Narrated `Amra bint `Abdur-Rahman:`Aisha, the wife of the Prophet (ﷺ) told her that once Allah's Messenger (ﷺ) was with her and she heard somebody asking permission to enter Hafsa's house. She said, "O Allah's Messenger (ﷺ)! This man is asking permission to enter your house." Allah's Messenger (ﷺ) replied, "I think he is so-and-so (meaning the foster uncle of Hafsa). What is rendered illegal because of blood relations, is also rendered illegal because of the corresponding foster-relations."
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ يُوسُفَ، أَخْبَرَنَا مَالِكٌ، عَنْ عَبْدِ اللَّهِ بْنِ أَبِي بَكْرٍ، عَنْ عَمْرَةَ ابْنَةِ عَبْدِ الرَّحْمَنِ، أَنَّ عَائِشَةَ، زَوْجَ النَّبِيِّ صلى الله عليه وسلم أَخْبَرَتْهَا أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم كَانَ عِنْدَهَا، وَأَنَّهَا سَمِعَتْ صَوْتَ إِنْسَانٍ يَسْتَأْذِنُ فِي بَيْتِ حَفْصَةَ فَقُلْتُ يَا رَسُولَ اللَّهِ، هَذَا رَجُلٌ يَسْتَأْذِنُ فِي بَيْتِكَ. فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " أُرَاهُ فُلاَنًا، لِعَمِّ حَفْصَةَ مِنَ الرَّضَاعَةِ، الرَّضَاعَةُ تُحَرِّمُ مَا تُحَرِّمُ الْوِلاَدَةُ ".
Bukhari
One-fifth of Booty to the Cause of Allah (Khumus) • Hadith 14