The Messenger of Allah ﷺ came in to see me while a man was sitting with me. This troubled him deeply, and I saw anger in his face. I said, "Messenger of Allah, he is my brother through breastfeeding."He said, "Examine carefully who your brothers are," and another time he said, "Consider carefully who your brothers through breastfeeding are, for breastfeeding only counts when it is from hunger."
أخبرنا هناد بن السري في حديثه عن أبي الأحوص عن أشعث بن أبي الشعثاء عن أبيه عن مسروق قال
دخل علي رسول الله ﷺ وعندي رجل قاعد فاشتد ذلك عليه ورأيت الغضب في وجهه فقلت يا رسول الله إنه أخي من الرضاعة فقال انظرن ما إخوانكن ومرة أخرى انظرن من إخوانكن من الرضاعة فإن الرضاعة من المجاعة
Narrated by Muslim, Book of Suckling, chapter "Suckling is only from hunger," hadith 1455, from Hannad. And by Bukhari, Book of Testimonies, chapter "Testimony regarding lineages, well-known suckling, and old death," hadith 2647, hadith 5102, from the hadith of Ashath, through the same chain. It is also found in Sunan al-Kubra, hadith 5463.