(2) باب تَحْرِيمِ الرَّضَاعَةِ مِنْ مَاءِ الْفَحْلِ
'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: Sahih Muslim 1445 i
In-book reference: Book 17, Hadith 12
USC-MSA web (English) reference(deprecated numbering scheme): Book 8, Hadith 3405