I heard Abu Salih say: ‘Ali ؓ said: I mentioned the daughter of Hamzah (as a potential spouse) to the Messenger of Allah ﷺ and he said: ʿShe is the daughter of my brother through breastfeeding.ʿ (Using translation from Aḥmad 1169)
ذَكَرْتُ بِنْتَ حَمْزَةَ لِرَسُولِ اللهِ ﷺ لِيَتَزَوَّجَها فَقَالَ «هِيَ ابْنَةُ أَخِي مِنَ الرَّضَاعَةِ»