Henry Drummond

"You will forgive me, I hope, for the sake of the friendship between us,
Which is too true and too sacred to be so easily broken!"

Longfellow

He that wrongs his friend
Wrongs himself more, and ever bears about
A silent court of justice in his breast,
Himself the judge and jury, and himself
The prisoner at the bar, ever condemn'd:
And that drags down his life.

Tennyson

Friendship is the marriage of the soul.

Voltaire