That which is lawful is graded into five classes. (1) Farẓ, that which is proved beyond all doubt to have been enjoined either in the Qurʾān or in a tradition of undoubted authority, and the denial or disobedience of which is positive infidelity. (2) Wājib, that which is obligatory, but of which there is some doubt whether or not it was enjoined in the Qurʾān or in a tradition of undoubted authority. (3) Sunnah, that which was practised by Muḥammad; (4) Mustāḥabb, that which Muḥammad and his Companions sometimes did and sometimes omitted; (5) Mubāḥ, that which is desirable, but which may be omitted without fear of sin.
Things which are unlawful are graded into three classes: (1) Mufsid, that which is most vicious and corrupting, a mortal sin; (2) Ḥarām, that which is distinctly forbidden; (3) Makrūh, that which is generally held to be unclean.
These distinctions of lawful and unlawful, with their various subdivisions, apply to all branches of Muslim law, whether it relate to ordinary duties of life, or of devotion to God.
It will be seen how important a place the example, practices, and sayings of Muḥammad occupy in the moral law of Islām. This branch of Muslim law is called as-Sunnah, or the custom of Muḥammad, and is distinguished as—
(1) Sunnatu ʾl-ʿfilī, that which Muḥammad himself did.
(2) Sunnatu ʾl-qaulī, that which Muḥammad said should be practised.
(3) Sunnatu ʾt-taqrīrī, that which was done in the presence of Muḥammad, and which he appears to have sanctioned.
It is therefore a serious mistake to suppose that the Qurʾān contains all that is esteemed necessary for faith and practice in Islām; the example of Muḥammad is as binding upon the Muslim as any injunction contained in the Qurʾān itself, for neither that which is Farẓ nor that which is Sunnah can be omitted without sin.
The true origin and fountain of all law is the Qurʾān and the Traditions, and no Muslim school of theology has ever rejected the Traditions. They are binding upon Sunnī, and Shīʿah, and Wahhābī; the only difference between the Sunnī and Shīʿah being that they receive different collections of Traditions. The Wahhābīs receive those of the Sunnīs, and call themselves Muḥaddis̤in, or traditionists.
In addition to the Qurʾān and Ḥadīs̤ (or Traditions), both Sunnī and Shīʿah Muslims acknowledge the concurrence of the learned, called Ijmāʿ, the Shīʿahs believing that they still possess Mujtahids capable of giving an infallible interpretation of the law; the Sunnīs, on the other hand, confessing that, since the days of the four great doctors (Abū Ḥanīfah, Mālik, ash-Shāfiʿī, and Ibn Ḥanbal), Ijmāʿ has not been possible; whilst the Wahhābīs accept only the Ijmāʿ of those who conversed with the Prophet himself. The fourth foundation of orthodoxy in both Sunnī and Shīʿah schools is the system of interpretation called Qiyās, or ratiocination.