From Sahih Muslim (Hadith 3579):
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
Reference: Muslim 3579