Sahih Muslim Book 8, Hadith Number 3430.

Chapter: Suckling of a young (boy).

‘Aisha (Allah be pleased with her) reported: Allah’s Messenger (May peace be upon him) visited me when a man was sitting near me, and he seemed to disapprove of that. And I saw signs of anger on his face and I said: Messenger of Allah, he is my brother by fosterage, whereupon he said: Consider who your brothers are because of fosterage since fosterage is through hunger (i.e. in infancy).