Musnad Ahmad 3:117

Ya'qub narrated to us, my father narrated to us, from Salih. Ibn Shihab said: Yahya ibn Sa'id ibn al-'As informed me that Sa'id ibn al-'As informed him: That 'Uthman and 'Aishah narrated to him: That Abu Bakr sought permission to enter upon the Messenger of Allah (may Allah bless him and grant him peace) while he was lying on his bed, wearing 'Aishah's garment... then he mentioned the meaning of the hadith of 'Uqayl.