Sahih Muslim
The Book of Marriage (Kitab Al-Nikah)
Book Number: 008 / Hadith Number: 3405
'A'isha (Allah be pleased with her) reported that her foster-uncle whose name was Aflah sought permission from her (to enter the house) but she observed seclusion from him, and informed Allah's Messenger (may peace be upon him) who said to her: Don't observe veil from him for he is Mahram (one with whom marriage cannot be contracted) on account of fosterage as one is Mahram on account of consanguinity.