Musannaf Ibn Abi Shayba 33:279

Ali said: "Words, if you were to travel the mounts for them, you would have exhausted them before you reached the likes of them: Let no servant hope except in his Lord, and let him not fear except his sin. Let not one who does not know be ashamed to learn, and let not a scholar be ashamed, when he is asked about what he does not know, to say: 'Allah knows best.' And know that the position of patience in relation to faith is like the position of the head in relation to the body; if the head goes, the body goes, and if patience goes, faith goes."