كتاب الرضاع
17  
The Book of Suckling
(2)
Chapter: The prohibition that results from breastfeeding is related to the issue of the male
(2)
باب تَحْرِيمِ الرَّضَاعَةِ مِنْ مَاءِ الْفَحْلِ ‏
Sahih Muslim 1445 i

'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.
حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، حَدَّثَنَا لَيْثٌ، ح وَحَدَّثَنَا مُحَمَّدُ بْنُ رُمْحٍ، أَخْبَرَنَا اللَّيْثُ، عَنْ يَزِيدَ بْنِ أَبِي حَبِيبٍ، عَنْ عِرَاكٍ، عَنْ عُرْوَةَ، عَنْ عَائِشَةَ، أَنَّهَا أَخْبَرَتْهُ أَنَّ عَمَّهَا مِنَ الرَّضَاعَةِ - يُسَمَّى أَفْلَحَ - اسْتَأْذَنَ عَلَيْهَا فَحَجَبَتْهُ فَأَخْبَرَتْ رَسُولَ اللَّهِ صلى الله عليه وسلم فَقَالَ لَهَا ‏ "‏ لاَ تَحْتَجِبِي مِنْهُ فَإِنَّهُ يَحْرُمُ مِنَ الرَّضَاعَةِ مَا يَحْرُمُ مِنَ النَّسَبِ ‏"‏ ‏.‏
Reference : Sahih Muslim 1445i
In-book reference : Book 17, Hadith 12
USC-MSA web (English) reference : Book 8, Hadith 3405
  (deprecated numbering scheme)