Sahih Bukhari Volume 7, Book 67, Hadith 5101

حَدَّثَنَا إِسْمَاعِيلُ، قَالَ حَدَّثَنِي مَالِكٌ، عَنْ عَبْدِ اللَّهِ بْنِ أَبِي بَكْرٍ، عَنْ عَمْرَةَ بِنْتِ عَبْدِ الرَّحْمَنِ، أَنَّ عَائِشَةَ، زَوْجَ النَّبِيِّ صلى الله عليه وسلم أَخْبَرَتْهَا أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم كَانَ عِنْدَهَا، وَأَنَّهَا سَمِعَتْ صَوْتَ رَجُلٍ يَسْتَأْذِنُ فِي بَيْتِ حَفْصَةَ، قَالَتْ فَقُلْتُ يَا رَسُولَ اللَّهِ هَذَا رَجُلٌ يَسْتَأْذِنُ فِي بَيْتِكَ‏.‏ فَقَالَ النَّبِيُّ صلى الله عليه وسلم ‏"‏ أُرَاهُ فُلاَنًا ‏"‏‏.‏ لِعَمِّ حَفْصَةَ مِنَ الرَّضَاعَةِ‏.‏ قَالَتْ عَائِشَةُ لَوْ كَانَ فُلاَنٌ حَيًّا، لِعَمِّهَا مِنَ الرَّضَاعَةِ دَخَلَ عَلَىَّ فَقَالَ ‏"‏ نَعَمِ الرَّضَاعَةُ تُحَرِّمُ مَا تُحَرِّمُ الْوِلاَدَةُ ‏"‏‏

Narrated By 'Aisha : (The wife of the Prophet) that while Allah's Apostle was with her, she heard a voice of a man asking permission to enter the house of Hafsa. 'Aisha added: I said, "O Allah's Apostle! This man is asking permission to enter your house." The Prophet said, "I think he is so-and-so," naming the foster-uncle of Hafsa. 'Aisha said, "If so-and-so," naming her foster uncle, "were living, could he enter upon me?" The Prophet said, "Yes, for foster suckling relations make all those things unlawful which are unlawful through corresponding birth (blood) relations."