Musnad Ahmad 1096
It was narrated that Sa’eed bin al-Musayyab said:
‘Ali (رضي الله عنه) said: I said: O Messenger of Allah (ﷺ), shall I not tell you about the most beautiful girl of Quraish? He said: “who is she?` I said: The daughter of Hamzah. He said: `Don`t you know that she is the daughter of my brother through breastfeeding? Allah has forbidden (for marriage) through breastfeeding that which He has forbidden through blood ties.”
حَدَّثَنَا وَكِيعٌ، حَدَّثَنَا سُفْيَانُ، عَنْ عَلِيِّ بْنِ زَيْدٍ، عَنْ سَعِيدِ بْنِ الْمُسَيَّبِ، قَالَ قَالَ عَلِيٌّ قُلْتُ يَا رَسُولَ اللَّهِ أَلَا أَدُلُّكَ عَلَى أَجْمَلِ فَتَاةٍ فِي قُرَيْشٍ قَالَ وَمَنْ هِيَ قُلْتُ ابْنَةُ حَمْزَةَ قَالَ أَمَا عَلِمْتَ أَنَّهَا ابْنَةُ أَخِي مِنْ الرَّضَاعَةِ إِنَّ اللَّهَ حَرَّمَ مِنْ الرَّضَاعَةِ مَا حَرَّمَ مِنْ النَّسَبِ‏.‏