Musannaf Ibn Abi Shayba 37:468

Sharik narrated to us, from Shabib ibn Gharqada, from al-Mustazill ibn Husayn, who said: Ali said: "O people of Kufa, you shall certainly enjoin the good and you shall certainly forbid the evil, and you shall certainly strive in the command of Allah, or else there will be people among you who will punish you, and Allah will punish them."