Musnad Ahmad 937:171

Yazid and Muhammad ibn Yazid narrated to us, they said: Al-Awwam narrated to us, Muhammad said: From Al-Qasim, and Yazid said in his narration: Al-Qasim ibn Awf al-Shaybani narrated to me, from a man who said: We had carried something for Abu Dharr, wanting to give it to him. We came to al-Rabadhah and asked about him, but we did not find him. It was said: He requested permission for Hajj, and it was granted to him. So we came to him in the town, and it was Mina. While we were with him, it was said to him: "Indeed, Uthman prayed four [rak'ahs]." This was difficult for Abu Dharr, and he spoke harshly and said: "I prayed with the Messenger of Allah, peace and blessings of Allah be upon him, and he prayed two rak'ahs. And I prayed with Abu Bakr and Umar." Then Abu Dharr stood up and prayed four. It was said to him: "You criticized the Commander of the Believers for something, then you did it yourself?" He said: "Dissent is more severe. Indeed, the Messenger of Allah, peace and blessings of Allah be upon him, addressed us and said: 'There will be a ruler after me, so do not humiliate him. Whoever wants to humiliate him has removed the yoke of Islam from his neck, and no repentance will be accepted from him until he repairs the breach he has caused—and he will not do so—then he returns and becomes among those who honor him.'" The Messenger of Allah, peace and blessings of Allah be upon him, commanded us not to let them overcome us regarding three things: that we enjoin the good, forbid the evil, and teach the people the Sunnahs.