From Sahih Muslim (Hadith 3585):
Umm Salama (Allah be pleased with her), the wife of Allah's Apostle (ﷺ), said:It was said to the Messenger of Allah (ﷺ): 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
Reference: Muslim 3585