Chapter 18: The Book of Suckling
' 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 (ﷺ) 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.
Sahih Muslim 1445i