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.  

سَمِعْتُ أُمَّ سَلَمَةَ زَوْجَ النَّبِيِّ ﷺ تَقُولُ قِيلَ لِرَسُولِ اللَّهِ ﷺ أَيْنَ أَنْتَ يَا رَسُولَ اللَّهِ عَنِ ابْنَةِ حَمْزَةَ أَوْ قِيلَ أَلاَ تَخْطُبُ بِنْتَ حَمْزَةَ بْنِ عَبْدِ الْمُطَّلِبِ قَالَ إِنَّ حَمْزَةَ أَخِي مِنَ الرَّضَاعَةِ  

[Machine] I heard Umm Salamah saying that it was said, "O Messenger of Allah, where are you from Bint Hamzah?" Or it was said, "Don't you propose to Bint Hamzah?" He said, "Hamzah is my brother through breastfeeding."  

سَمِعْتُ أُمَّ سَلَمَةَ تَقُولُ قِيلَ يَا رَسُولَ اللهِ أَيْنَ أَنْتَ مِنْ بِنْتِ حَمْزَةَ؟ أَوْ قِيلَ أَلَا تَخْطُبُ بِنْتَ حَمْزَةَ؟ قَالَ «حَمْزَةُ أَخِي مِنَ الرَّضَاعَةِ»