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 لَوْ تَزَوَّجْتَ بِنْتَ حَمْزَةَ؟ قَالَ إِنَّهَا ابْنَةُ أَخِي مِنَ الرَّضَاعَةِ