Sahih Muslim Book 31, Hadith Number 5907.
‘Aisha, the wife of Allah’s Apostle (may peace be upon him), and Uthman both reported that Abu Bakr sought permission from Allah’s Messenger (may peace be upon him) for entrance (in his apartment) as he had been lying on his bed covered with the bed-sheet of ‘Aisha, and he gave permission to Abu Bakr in that very state and he, having his need fulfilled, went back. Then Umar sought permission and it was given to him in that very state and, after having his need fulfilled, he went back. And ‘Uthman reported: Then I sought permission from him and he got up and raid to ‘Aisha: Wrap yourself well with your cloth, then I got my need fulfilled and came back. And ‘Aisha said: Allah’s Messenger, why is it that I did not see you feeling any anxiety in case of dressing properly in the presence of Abu Bakr and ‘Umar (Allah be pleased with them) as you showed in case of ‘Uthman. Thereupon Allah’s Messenger (may peace be upon him) said: Verily Uthman is a person who is very modest and I was afraid that if I permitted him to enter in this very state he would not inform me of his need.