Musnad Ahmad 1153:1198

Hajjaj narrated to us, Laith narrated to us, Aqil narrated to me, from Ibn Shihab, from Yahya bin Sa'id bin al-As, that Sa'id bin al-As informed him: That Aisha, the wife of the Prophet, may Allah bless him and grant him peace, and Uthman narrated to him that Abu Bakr asked permission to enter upon the Messenger of Allah, may Allah bless him and grant him peace, while he was lying on his bed, wearing Aisha’s garment. He gave permission to Abu Bakr while he was in that state, so he fulfilled his need with him, then he departed. Then Umar asked permission, and he gave him permission while he was in that same state, so he fulfilled his need with him, then he departed. Then Uthman came and asked permission to enter upon him, so he sat up and said to Aisha: "Gather your clothes upon yourself." So he fulfilled his need with him, then he departed. Aisha said: "O Messenger of Allah, why did I not see you show concern for Abu Bakr and Umar as you showed concern for Uthman?" The Messenger of Allah, may Allah bless him and grant him peace, said: "Indeed, Uthman is a shy man, and I feared that if I gave him permission while in that state, he would not convey his need to me." Laith said: And a group of people said: The Messenger of Allah, may Allah bless him and grant him peace, said to Aisha: "Should I not be shy before a man whom the angels are shy before?"