Musannaf Ibn Abi Shayba 28:4

Waki’ narrated to us, [saying] Sufyan narrated to us, from Isma’il bin Abi Khalid, who said: I heard Mus’ab bin Sa’d saying: Ali bin Abi Talib said words in which he hit upon the truth: It is a duty upon the Imam to judge by what Allah has revealed and to fulfill the trust. If he does that, then it is a duty upon the Muslims to listen, obey, and respond when they are called.