Musannaf Ibn Abi Shayba 37:577

Ghundar narrated to us, from Shu'bah, from 'Amr ibn Murrah, who said: I heard Dhakwan Abu Salih narrating from Suhayb, the freed slave of al-'Abbas, who said: Al-'Abbas sent me to 'Uthman to summon him. He said: So I came to him, and he was feeding the people. I summoned him, and he came to him and said: "May the face prosper, O Abu al-Fadl." He said: "And your face, Commander of the Faithful." He said: "Nothing delayed me but that your messenger came to me while I was feeding the people, so I fed them, then I came." Al-'Abbas said: "I adjure you by Allah regarding 'Ali, for he is your cousin, your brother in your religion, your companion with the Messenger of Allah (may Allah bless him and grant him peace), and your in-law. It has reached me that you intend to take a stand against 'Ali and his companions, so excuse me from that, O Commander of the Faithful." 'Uthman said: "I am more entitled than your brother to have accepted your intercession. If 'Ali had wished, no one would have been above him, but he refused except to follow his own opinion." Then he sent for 'Ali and said to him: "I adjure you by Allah regarding your cousin, the son of your paternal aunt, your brother in your religion, your companion with the Messenger of Allah (may Allah bless him and grant him peace), and the guardian of your pledge of allegiance." He ('Ali) said: "By Allah, if he ordered me to leave my house, I would leave. But as for compromising so that the Book of Allah is not upheld, I would not do that." Muhammad ibn Ja'far said: I heard it from him more times than I can count, and I presented it to him more than once.