Narrated `Aisha:My foster uncle came and asked permission (to enter) but I refused to admit him till I asked Allah's Apostle about that. He said, "He is your uncle, so allow him to come in." I said, "O Allah's Messenger (ﷺ)! I have been suckled by a woman and not by a man." Allah's Messenger (ﷺ) said, "He is your uncle, so let him enter upon you." And that happened after the order of Al-Hijab (compulsory veiling) was revealed. All things which become unlawful because of blood relations are unlawful because of the corresponding foster suckling relations.
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ يُوسُفَ، أَخْبَرَنَا مَالِكٌ، عَنْ هِشَامِ بْنِ عُرْوَةَ، عَنْ أَبِيهِ، عَنْ عَائِشَةَ ـ رضى الله عنها ـ أَنَّهَا قَالَتْ جَاءَ عَمِّي مِنَ الرَّضَاعَةِ فَاسْتَأْذَنَ عَلَىَّ فَأَبَيْتُ أَنْ آذَنَ لَهُ حَتَّى أَسْأَلَ رَسُولَ اللَّهِ صلى الله عليه وسلم فَجَاءَ رَسُولُ اللَّهِ صلى الله عليه وسلم فَسَأَلْتُهُ عَنْ ذَلِكَ فَقَالَ " إِنَّهُ عَمُّكِ فَأْذَنِي لَهُ " قَالَتْ فَقُلْتُ يَا رَسُولَ اللَّهِ إِنَّمَا أَرْضَعَتْنِي الْمَرْأَةُ وَلَمْ يُرْضِعْنِي الرَّجُلُ. قَالَتْ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " إِنَّهُ عَمُّكِ فَلْيَلِجْ عَلَيْكِ ". قَالَتْ عَائِشَةُ وَذَلِكَ بَعْدَ أَنْ ضُرِبَ عَلَيْنَا الْحِجَابُ. قَالَتْ عَائِشَةُ يَحْرُمُ مِنَ الرَّضَاعَةِ مَا يَحْرُمُ مِنَ الْوِلاَدَةِ.
Bukhari
Wedlock, Marriage (Nikaah) • Hadith 172