Musnad Ahmad 4:20

Abdullah narrated to us, Sufyan ibn Waki’ narrated to me, Qabisah narrated to me, from Abu Bakr ibn Ayyash, from Asim, from Abu Wa’il, who said: I said to Abd al-Rahman ibn Awf: "How did you pledge allegiance to Uthman and leave Ali?" He said: "What is my fault? I began with Ali and said: 'I pledge allegiance to you upon the Book of Allah, the Sunnah of His Messenger, and the conduct of Abu Bakr and Umar.' He said: 'To the extent of my ability.'" He said: "Then I presented it to Uthman, and he accepted it."