Sahih Muslim 1448
Sahih Muslim · Book 17: The Book of Suckling · The daughter of one's brother through breastfeeding is forbidden in marriage
Umm Salama (Allah be pleased with her), the wife of Allah's Apostle (ﷺ), said:
وَحَدَّثَنَا هَارُونُ بْنُ سَعِيدٍ الأَيْلِيُّ ، وَأَحْمَدُ بْنُ عِيسَى ، قَالاَ حَدَّثَنَا ابْنُ وَهْبٍ ، أَخْبَرَنِي مَخْرَمَةُ بْنُ بُكَيْرٍ ، عَنْ أَبِيهِ ، قَالَ سَمِعْتُ عَبْدَ اللَّهِ بْنَ مُسْلِمٍ ، يَقُولُ سَمِعْتُ مُحَمَّدَ بْنَ مُسْلِمٍ ، يَقُولُ سَمِعْتُ حُمَيْدَ بْنَ عَبْدِ الرَّحْمَنِ ، يَقُولُ سَمِعْتُ أُمَّ سَلَمَةَ ، زَوْجَ النَّبِيِّ صلى الله عليه وسلم تَقُولُ قِيلَ لِرَسُولِ اللَّهِ صلى الله عليه وسلم أَيْنَ أَنْتَ يَا رَسُولَ اللَّهِ عَنِ ابْنَةِ حَمْزَةَ . أَوْ قِيلَ أَلاَ تَخْطُبُ بِنْتَ حَمْزَةَ بْنِ عَبْدِ الْمُطَّلِبِ قَالَ " إِنَّ حَمْزَةَ أَخِي مِنَ الرَّضَاعَةِ " .
It was said to the Messenger of Allah (ﷺ): Is not the daughter of Hamza a suitable match for you? Or it was said: Why don't you propose to marry the daughter of Hamza, the son of Abd al-Muttalib? Thereupon he said: Hamza is my brother by reason of fosterage.