Sunan an-Nasa'i 3312

It was narrated that Masruq said:"Aishah said: 'The Messenger of Allah entered upon me and there was a man sitting with me. He got upset about that, and I saw the anger in his face.' I said: "O Messenger of Allah, he is my brother through breast-feeding." He said: "Be careful who you count as your brothers" --or: "be careful who you count as your brothers through breast-feeding"-- "for the breast-feeding (which makes marriage prohibited) is from hunger