Emerson

A good man is the best friend, and therefore soonest to be chosen, longer to be retained, and, indeed, never to be parted with, unless he cease to be that for which he was chosen.

Jeremy Taylor

Friendship sealed by companionship in sin will not last long.

Arnot

Eschew that friend, if thou art wise,
Who consorts with thy enemies.

Saadi

Friendship requires a steady, constant, and unchangeable character, a person that is uniform in his intimacy.

Plutarch