Musnad Ahmad 1153:553

Abu al-Mughirah narrated to us, he said: al-Walid ibn Sulayman narrated to us, he said: Rabi'ah ibn Yazid narrated to me, from Abdullah ibn Amir, from al-Nu'man ibn Bashir, from Aisha, who said: The Messenger of Allah (peace and blessings of Allah be upon him) sent for Uthman ibn Affan, and the Messenger of Allah (peace and blessings of Allah be upon him) turned toward him. When we saw the Messenger of Allah (peace and blessings of Allah be upon him), we turned to one another. Among the last words he spoke to him was that he struck his shoulder and said: "O Uthman, it may be that Allah, the Almighty and Majestic, will clothe you in a shirt. If the hypocrites want you to take it off, do not take it off until you meet me. O Uthman, it may be that Allah will clothe you in a shirt. If the hypocrites want you to take it off, do not take it off until you meet me." He said it three times. I said to her: "O Mother of the Believers, why did you keep this from us?" She said: "I forgot it, by Allah, and I did not remember it." He said: So I informed Mu'awiyah ibn Abi Sufyan of it, and he was not satisfied with what I told him until he wrote to the Mother of the Believers, saying: "Write it down for me." So she wrote it down for him in a letter.