Musannaf Ibn Abi Shayba 37:146

Ali ibn Mushir narrated to us, from al-Shaybani, from Abdullah ibn al-Mukharraq ibn Sulaym, from his father, who said: Ali said: "I do not see these people except that they will prevail over you due to your disunity regarding your truth and their unity upon their falsehood. And the Imam does not have his journey made difficult, and he makes mistakes and is correct. So if you have an Imam who acts justly among the subjects and distributes equally, then listen to him and obey him. For the people, nothing will set them right except an Imam, whether righteous or wicked. If he is righteous, then it is for the shepherd and the flock. And if he is wicked, the believer worships his Lord in it, and the wicked one works in it until his appointed time. And you will be presented with cursing me and disavowing me. So whoever curses me, he is permitted to curse me, but do not disavow my religion, for I am upon Islam."