The Prophet ﷺ said about Hamza's daughter, "I am not legally permitted to marry her, as foster relations are treated like blood relations (in marital affairs). She is the daughter of my foster brother."  

قَالَ النَّبِيُّ ﷺ فِي بِنْتِ حَمْزَةَ لاَ تَحِلُّ لِي يَحْرُمُ مِنَ الرَّضَاعِ مَا يَحْرُمُ مِنَ النَّسَبِ هِيَ بِنْتُ أَخِي مِنَ الرَّضَاعَةِ