Pope

Keep thy friend under thy own life's key.

Shakespeare

Greater love hath no man than this, that a man lay down his life for his friends.

John xv. 13

The friendship of the pure-minded, whether in presence or absence, is not such that they will find fault with thee behind thy back, and die for thee in thy presence.

Saadi

"Yes, we must ever be friends; and of all who offer you friendship
Let me be ever the first, the truest, the nearest and dearest!"

Longfellow