Sahih Muslim Book 8, Hadith Number 3411.

Chapter: Kitab Al-Rada’a-Book pertaining to fosterage.

Umm Salama (Allah be pleased with her), the wife of Allah’s Apostle (May peace be upon him), said: It was said to the Messenger of Allah (May peace be upon him): Is not the daughter of Hamza a suitable match for you? Or it was said: Why don’t you propose to marry the daughter of Hamza, the son of Abd al-Muttalib? Thereupon he said: Hamza is my brother by reason of fosterage.