Musnad Ahmad 1153:244

Yahya narrated to us, from Isma'il, who said: Qays narrated to us, from Abu Sahlah, from Aisha, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Call some of my companions for me." I said: "Abu Bakr?" He said: "No." I said: "Umar?" He said: "No." I said: "Your cousin Ali?" He said: "No." She said: I said: "Uthman?" He said: "Yes." When he came, he said: "Move aside." He began whispering to him, and the color of Uthman’s face changed. When the day of the house (the siege) came and he was besieged in it, we said: "O Commander of the Faithful, will you not fight?" He said: "No, for the Messenger of Allah (peace and blessings of Allah be upon him) made a covenant with me regarding a matter, and I am patient with myself upon it."