Messenger of Allah, he is the person who seeks permission to enter your house, whereupon Messenger of Allah ﷺ said: I think he is so and so (uncle of Hafsa by reason of fosterage). 'A'isha said: Messenger of Allah, if so and so (her uncle by reason of fosterage) were alive, could he enter my house? Messenger of Allah ﷺ said: Yes. Fosterage makes unlawful what consanguinity makes unlawful.  

أَنَّ رَسُولَ اللَّهِ ﷺ كَانَ عِنْدَهَا وَإِنَّهَا سَمِعَتْ صَوْتَ رَجُلٍ يَسْتَأْذِنُ فِي بَيْتِ حَفْصَةَ قَالَتْ عَائِشَةُ فَقُلْتُ يَا رَسُولَ اللَّهِ هَذَا رَجُلٌ يَسْتَأْذِنُ فِي بَيْتِكَ فَقَالَ رَسُولُ اللَّهِ ﷺ أُرَاهُ فُلاَنًا لِعَمِّ حَفْصَةَ مِنَ الرَّضَاعَةِ فَقَالَتْ عَائِشَةُ يَا رَسُولَ اللَّهِ لَوْ كَانَ فُلاَنٌ حَيًّا لِعَمِّهَا مِنَ الرَّضَاعَةِ دَخَلَ عَلَىَّ قَالَ رَسُولُ اللَّهِ ﷺ نَعَمْ إِنَّ الرَّضَاعَةَ تُحَرِّمُ مَا تُحَرِّمُ الْوِلاَدَةُ  

"What becomes unlawful through breastfeeding is that which becomes unlawful through birth."  

لِي رَسُولُ اللَّهِ ﷺ يَحْرُمُ مِنَ الرَّضَاعَةِ مَا يَحْرُمُ مِنَ الْوِلاَدَةِ  

The above hadith is narrated through another chain.  

أَخْبَرَنِي عَبْدُ اللَّهِ بْنُ أَبِي بَكْرٍ بِهَذَا الإِسْنَادِ مِثْلَ حَدِيثِ هِشَامِ بْنِ عُرْوَةَ