Musnad Abu Ya'la al-Mawsili 150:301

Musa ibn Muhammad ibn Hayyan narrated to us, Yahya narrated to us, from Isma'il ibn Abi Khalid, from Qays, from Abu Sahlah, from 'Aishah, 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." I said: "Who?" He said: "Uthman." When he came, he said: "Move aside." Then he began whispering to him, and the color of Uthman began to change. When the day of the house (the siege) came and he was besieged, we said: "O Commander of the Faithful, will you not fight?" He said: "The Messenger of Allah (peace and blessings of Allah be upon him) made a covenant with me regarding a matter, and I am restraining myself for it."