Sahih Muslim Book 8, Hadith Number 3397.

Chapter: Kitab Al-Rada’a-Book pertaining to fosterage.

‘Aisha (Allah be pleased with her) reported that Aflah, the brother of Abu’l-Qu’ais, who was her uncle by reason of fosterage, came, and asked her permission (to enter the house) after seclusion was instituted. I refused to admit him. When Allah’s Messenger (May peace be upon him) came, I Informed him what I had done. He commanded me to grant him permission (as the brother of her foster-father was also her uncle).