Musnad Ahmad 3:103

Waki’ narrated to us, from Isma’il ibn Abi Khalid, who said: Qays said: Abu Sahlah narrated to me: That ‘Uthman said on the day of the house [the siege], when he was besieged: "The Prophet, peace and blessings of Allah be upon him, entrusted a covenant to me, and I am patient regarding it." Qays said: And they used to consider it [the fulfillment of] that on that day.