Musnad Abu Ya'la al-Mawsili 3:258

Isma'il ibn Musa narrated to us, al-Rabi' ibn Sahl al-Fazari narrated to us, Sa'id ibn 'Ubayd narrated to me, from 'Ali ibn Rabi'ah, who said: I heard 'Ali upon the pulpit, and a man came to him and said: "O Commander of the Faithful, why do I see you treating the people as a man treats his camels? Is it by a covenant from the Messenger of Allah (may Allah bless him and grant him peace), or something you have seen?" He said: "By Allah, I have not lied, nor have I been lied to; I have not gone astray, nor has anyone gone astray through me. Rather, it is a covenant from the Messenger of Allah (may Allah bless him and grant him peace) that he entrusted to me, and he who fabricates has failed."