Sahih Muslim Book 8, Hadith Number 3409.

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

Ibn Abbas (Allah be pleased with them) reported: It was proposed that he (the Holy Prophet) be married to the daughter of Hamza, whereupon he said: She is not lawful for me for she is the daughter of my foster-brother, and that is unlawful by reason of fosterage what is unlawful by reason of genealogy.