The best teachings in the Koran are borrowed from the Bible or the Talmud. In consequence of the difficulties which Mahomet for several years (612–633) had to encounter in Mecca on account of these purified doctrines, there grew around the sound kernel a loathsome husk. Mahomet's connection with the Jews of Arabia assisted not a little in determining and modifying the teachings of Islam. Portions of the Koran are devoted to them, at times in a friendly, at times in a hostile spirit.

When Mahomet failed in obtaining a hearing in Mecca, the seat of idolatrous worship in Arabia, and even ran the risk of losing his life there, he addressed himself to some men from Yathrib, and urged them to accept his doctrines. These men were more familiar with Jewish doctrines than the Meccans; they found in Mahomet's revelations a close analogy to what they had often heard from their Jewish neighbors. They, therefore, showed themselves inclined to follow him, and caused him to be invited to Yathrib, where his teachings were likely to be favorably received on account of the numerous Jews residing there. As soon as he came there (622, the year of expatriation—Hejira), Mahomet took care to win over the Jews of Yathrib and to set forth his aims, as though he desired to bring about the universal recognition of Judaism in Arabia. When he saw the Jews fasting on the day of Atonement, he said, "It becomes us more than Jews to fast on this day," and he established a fast-day (Ashura). Mahomet entered into a formal alliance for mutual defense with the Jewish tribes, and instituted the custom of turning towards Jerusalem in prayer (Kiblah). In the disputes between the Jews and his disciples (Moslems), which were submitted to his judgment, he behaved leniently to the Jews. For this reason Mahomet's disciples preferred to bring the matters in dispute before a Jewish chief, because they expected more impartiality from him than from Mahomet. Mahomet for a long time employed a Jewish scribe to do his correspondence, he himself being unable to write. These advances on the part of a man of so much promise were very flattering to the Jews of Medina. They looked upon him to some extent as a Jewish proselyte, and expected to see Judaism through him attain to power in Arabia. Some of them followed him devotedly and were his faithful allies (Ansar); amongst them was a learned youth, Abdallah Ibn-Salâm, of the race of Kainukaa. Abdallah and other Jews assisted Mahomet in propagating the Koran. The unbelieving Arabs frequently reproached him, saying that he was an ear (accepted anything as truth), that it was not the angel Gabriel who was teaching him, but a mortal man. Nevertheless, though Abdallah Ibn-Salâm and other Jewish Ansars supported him, they were far from abandoning Judaism on this account, and continued to observe the Jewish commandments, and Mahomet was at first not offended by this conduct.

But only a small number of the Jews of Medina joined the band of believers, particularly when they perceived his selfish efforts, his haughtiness, and his insatiable love of women. They bore in their hearts too high an ideal of their ancient prophets to place this enthusiast, who longed after every beautiful woman, on an equal footing with them. "See him," said the Jews, "he is not satisfied with food, and has no other desire than that of being surrounded by women. If he is a prophet, he should confine himself to his duties as a prophet, and not turn to women." Other Jews said: "If Mahomet is a prophet, he should appear in Palestine, for only in that place God appears unto his elect." The Jews also objected to him, saying, "You pride yourself on being of Abraham's faith, but Abraham did not use the flesh and milk of camels." Mahomet's chief opponents on the Jewish side were Pinehas Ibn-Azura, a man of caustic wit, who seized every opportunity to make Mahomet appear ridiculous; furthermore, the far-famed Kaab Ibn-Asharaf, the offspring of an Arab father and a Jewish mother; a poet, Abu-Afak, an old man more than a hundred years old, who endeavored to arouse hate against Mahomet amongst the ignorant Arabs; and Abdallah, the son of Saura, who was looked upon as the most learned Jew in Hejas. Pinehas is the author of a witty answer to Mahomet's invitation to the Jewish tribe of Benu-Kainukaa to accept Islam. Mahomet, in his epistle, had used the words: "Lend yourselves unto God as a beautiful pledge." Pinehas answered, "God is so poor that He borrows from us!" Thus the Jewish opponents of Mahomet placed a ridiculous meaning on his sayings and revelations, and treated him contemptuously, not anticipating that the fugitive from Mecca, who had come to Medina for assistance, would shortly humble and in part destroy their tribes, and that he would control the destiny of many of their co-religionists in times to come. They relied too much on their own courage and strength, and forgot that the most dangerous enemy is he whom one disregards too much. Mahomet, indeed, with sly dissimulation, at first accepted the contempt bestowed on him by the Jews with apparent equanimity. He advised his disciples, "Fight only in a becoming manner with the people who believe in the Holy Writ (Jews), and say: We believe in that which has been revealed to us and to you. Our God is the same as yours, and we are faithful to Him." But the mutual discontent made it difficult to maintain peace permanently. On the one side, the Jews did their best to alienate Mahomet's followers. They succeeded in prejudicing the first man in Medina, the Chazrajite Abdallah Ibn-Ubey, against Mahomet, so that he remained antagonistic to Mahomet to the end of his days. This man was about to be elected king of his town, but through the arrival of Mahomet he had been cast into the shade. On the other side, his followers urged him to declare to what extent he held to Judaism. They saw that his disciples amongst the Jews still continued to observe the Jewish laws, and to abstain from camel's flesh, and they said to him, "If the Torah be a divine book, then let us follow its teachings." Since Mahomet was thoroughly an Arab, he could not join Judaism, and he perceived that the Arabs would not conform to religious customs which were quite strange to them. So it only remained for him to break with the Jews definitely. He thereupon published a long Sura (called the Sura of the Cow), full of invectives against the Jews. He altered the position assumed in prayer, and decreed that the believers should no longer turn their faces towards Jerusalem, but towards Mecca and the Kaaba. He discarded fasting on the day of Atonement (Ashura), and instituted instead the holy month Ramadhan, as had been customary among the Arabs from very ancient times. He was obliged to withdraw much of what he had in the beginning given out as God's revelation. Mahomet now asserted that the Torah had contained many allusions to his appearance and calling as a prophet, but that the Jews had expunged the passages. At first he declared that the Jews were possessed of the true faith; later on he said that they honored Ezra (Ozaïr) as the son of God, just as the Christians did Jesus, and that the Jews were consequently to be regarded as infidels. His hatred against the Jews, who refused to accept his prophecies, and saw through his designs, continually widened the breach between them and him.

Although he hated the Jews in his innermost heart, yet he did not venture to provoke them by acts of violence, because his authority was not sufficiently great, and the Jews outnumbered his followers. But after the battle at Bedr (in the winter of 624), when the small body of Mahometans gained a victory over the numerous Koraishites, the situation changed. Mahomet, whose power was greatly increased through this victory, exchanged the attitude of a humble prophet for that of a fanatical tyrant, to whom any measure, even assassination, was a justifiable means of freeing himself from his enemies. However, he was prudent enough to avoid becoming involved in disputes with the powerful Jewish tribes; he began with the weak and defenseless. A poetess, Asma, daughter of Merwan, who was of Jewish descent, and married to an Arab, was murdered at night whilst asleep (because she had composed satires against the false prophet), and he commended the murderer. Thereupon the Jewish tribe Kainukaa experienced his religious wrath. It was the weakest of the Jewish-Arabian tribes, and to it belonged that Pinehas Ibn-Azura, whose sarcastic wit had made Mahomet appear in a ridiculous light. The pretext was of the slightest kind. A Mahometan had killed a Jew on account of a poor practical joke, and the Kainukaa avenged his death. Mahomet thereupon challenged them to profess Islam, or to accept war as the alternative. They replied: "We are, it is true, for peace, and would gladly maintain our alliance with you; but since you desire to make war upon us, we will show that we have no fear." They reckoned upon the assistance of the tribes of Nadhir and Kuraiza, who were their co-religionists, and withdrew to their fortresses at Medina. Mahomet collected his troops, and besieged the Kainukaa. Had the numerous Jews of northern Arabia, Nadhir, Kuraiza, and those of Chaibar, who, like the Kainukaa, were threatened, come to their assistance, and had they, before it was too late, made an offensive and defensive alliance, they would have been able to crush Mahomet and his straggling followers, on whose fidelity, moreover, he could not entirely rely. But the Jews, like the Arabs, were divided, and each tribe had only its own interests in view. The Kainukaa fought desperately for fifteen days, expecting re-inforcements from their co-religionists. But as these did not come, they surrendered to the enemy. Mahomet had all the Jews of Kainukaa put in chains with the intention of killing them; but a word from Abdallah Ibn-Ubey, their ally, made him draw back with alarm from his purpose. Abdallah laid hold of his shirt of mail, and said: "I will not let you go until you promise me to spare the captives; for they constitute my strength; they have defended me against the black people and the red people." To which Mahomet replied: "Let them be free; may God condemn them, and Abdallah with them!" The Jews of Kainukaa, 700 in number, were obliged to leave their possessions behind, and they set out for Palestine in a most destitute condition (February, 624). They settled in Batanea, whose chief town was Adraat, where they were probably received in a fraternal manner by their co-religionists, who, at this time, were free from the Byzantine yoke.

After the victory over the Kainukaa, Mahomet communicated to the Moslems a revelation against the Jews, which deprived them of every protection: "O ye believers, choose ye not Jews and Christians as allies; they may protect themselves. He who befriends them is one of them; God tolerates no sinful people." This exclusion was less harmful to the Christians, as they were not numerously represented in northern Arabia, and generally kept themselves neutral. The Jews, on the contrary, who were accustomed to independence, and who were full of warlike courage, became involved in numerous disputes by this act of outlawry. Their former allies for the most part renounced them, and at Mahomet's bidding, took spiteful vengeance on them.

With this mutual, deadly hatred existing between Mahomet and the Jews, it is said that the Benu-Nadhir invited him one day to their castle of Zuhara with the intention of hurling him from the terraces and thus ending his life. At that time their chief was Hujej Ibn-Achtab. Mahomet accepted the invitation, but watched the movements of the Jews. Suspecting that they desired his death, he stole away and hastened to Medina. The Jews of Nadhir paid dearly, it is said, for this treacherous project. Mahomet gave them the choice of quitting their homes within ten days, or of preparing for death. The Nadhir were resolved at first to avoid war and to emigrate, but encouraged by Abdallah, who promised them assistance, they accepted the challenge which had been thrown down. They, however, waited in vain for the assistance promised to them. Mahomet commenced operations against them, and uprooted and burnt the date-trees which supplied them with food. His own people rebelled at this proceeding, for to these unscrupulous warriors a palm was holier than a man's life. After several days of siege, the Nadhir were obliged to capitulate, and the terms were that they should depart without arms, and that they should take only a certain portion of their possessions—as much as a camel could carry.

They thereupon emigrated to the number of six hundred, some of them going to their countrymen in Chaibar, and some settling in Jericho and Adraat (June-July, 625). The war against the Nadhirites was, later on, justified by Mahomet through a revelation of the Koran, which read: "All in the heavens and earth praise God; He is the most honored, the most wise. He it is who drove out the unbelievers amongst the people of the Book from their dwelling places (Kainukaa), to send them to those who had already emigrated. You thought not that they would go forth, they themselves thought that their strong places would protect them from God himself, but God attacked them unexpectedly, and threw terror into their hearts, so that their houses were destroyed with their own hands, as well as laid waste by believers." The exiled Benu-Nadhir, who had remained in Arabia, did not accept their misfortune quietly, but exerted themselves to form a coalition with the enemies of Mahomet in order to attack him with combined forces. Three respected Nadhirites, Hujej, Kinanah Ibn-ol-Rabia, and Sallam Ibn Mishkam, incited the Koraishites in Mecca, in alliance with the mighty tribe of the Ghatafan and others, to make war against the haughty tyrannical prophet, who was daily becoming more powerful and more cruel. The enemies of Mahomet in Mecca, though filled with rage against him, were first incited by the Jews to join battle with him.

Through the activity of the Nadhirites the Arabian tribes were induced to join in the war. They found it more difficult, however, to induce their co-religionists, the Benu-Kuraiza, to take part. Kaab-Ibn-Assad, the governor of Kuraiza, at first would not receive the Nadhirite Hujej, who had desired his protection, because his tribe had made an alliance with Mahomet and the Moslems, and he was so guileless as to rely on Mahomet's word. Hujej managed to convince him of the danger which threatened the Jews, and to persuade him that the victory of so many allies over the less numerous Moslems was certain. The Benu-Kuraiza yielded to his arguments. Ten thousand of the allied troops took the field, and intended to surprise Medina. Mahomet, forewarned by a deserter, would not allow his army, which was inferior in numbers, to fight a pitched battle. He fortified Medina by surrounding it with a deep ditch and other defenses. The Arabs, accustomed to fight in single combat, vainly discharged their arrows against the fortifications. Mahomet succeeded finally in sowing the seeds of mutual distrust among the chief allies, viz., the Koraishites, the Ghatafan and the Jews.

The "War of the Fosse" terminated favorably for Mahomet, and very unhappily for the Jews, upon whom the whole of his wrath now fell. On the day after the departure of the allies, Mahomet, with 3000 men, took the field against Kuraiza, announcing that he was thus obeying an express revelation. His next step was to arouse the enthusiasm of his followers in the cause of the war. "Let him that is obedient offer up his prayers in the neighborhood of Kuraiza," was the formula with which he exhorted them. The Jews, unable to resist in a battle, retired to their fortresses, which they put into a state of defense. Here they were besieged by Mahomet and his troops for twenty-five days (February-March, 627). Food then began to fail the besieged, and it became necessary to think of capitulation. They besought Mahomet to treat them as he had treated their brethren, the Nadhirites, viz., allow them to withdraw with their wives, their children, and a portion of their property. The vindictive prophet, however, refused their request, and demanded unconditional surrender.

Nearly 700 Jews, amongst them the chiefs Kaab and Hujej, were ruthlessly slaughtered in the market-place, and their bodies thrown into a common grave. The market-place was thenceforth called the Kuraiza Place. And all this was done in the name of God! The Koran makes reference to it in the following verse: "God drove out of their fortresses those of the people of the Book [the Jews] who assisted the allies, and he cast into their hearts terror and dismay. Some of them you put to flight, some you took captive; he has caused you to inherit their land, their houses, and their wealth, and a land which you have not trodden; for God is almighty." The women were bartered for weapons and horses. Mahomet wished to retain one of the captives, a beautiful girl, Rihana by name, as his concubine; she, however, proudly rejected his advances. Only one of the Kuraiza remained alive, a certain Zabir Ibn-Bata, and he only by the intercession of Thabit, one of his friends. Full of joy, the latter hastened to the aged Zabir, to tell him of his fortune. "I thank thee," said the Jewish sage, who lay in fetters; "but tell me what has become of our leader Kaab?" "He is dead," answered Thabit. "And Hujej Ibn-Achtab, the prince of the Jews?" "He is dead," he again replied. "And Azzel Ibn-Samuel, the fearless warrior?" "He, too, is dead," was his answer again. "Then I do not care to live," said Zabir. The old man begged that he might die by the hands of his friend. His wish was granted.