Musnad Ahmad 1155:44

Rawh narrated to us, Ibn Jurayj narrated to us, he said: Abu Khalid informed me, from Abdullah ibn Abi Sa’id al-Madani, he said: Hafsa bint Umar ibn al-Khattab narrated to me, she said: The Messenger of Allah (peace and blessings of Allah be upon him) had one day placed his garment between his thighs. Abu Bakr came and asked for permission, and he permitted him while he was in that state. Then Umar came with a story like this, then Ali, then people from his companions, and the Prophet (peace and blessings of Allah be upon him) was in his state. Then Uthman came and asked for permission, and he permitted him, so he took his garment and covered himself with it, and they spoke, then they left. I said: "O Messenger of Allah, Abu Bakr, Umar, Ali, and the rest of your companions came, and you were in your state, but when Uthman came, you covered yourself with your garment?" He said: "Should I not be shy of a man whom the angels are shy of?"