It was said to the Prophet, "Won't you marry the daughter of Hamza?" He said, "She is my foster niece (brother's daughter). " (Using translation from Bukhārī 5100)

قِيلَ لِلنَّبِيِّ ﷺ أَلَا تَزَوَّجُ بِنْتَ حَمْزَةَ؟ قَالَ «إِنَّهَا بِنْتُ أَخِي مِنَ الرَّضَاعَةِ»