It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) was offered marriage to the daughter of Sayyiduna Hamzah (may Allah be pleased with him), so he (peace and blessings be upon him) said: "She is not lawful for me; she is the daughter of my brother through fosterage, and fosterage makes unlawful the same relations that lineage makes unlawful."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12767