It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both): It was said to the Prophet (peace and blessings be upon him): "Will you not marry the daughter of Sayyiduna Hamzah (may Allah be pleased with him)?" He (peace and blessings be upon him) said: "She is the daughter of my brother through fosterage."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12768