Ayesha reported that the Messenger of Allah ﷺ was with her when she heard a man asking permission to enter Hafsa's house.Ayesha said, "Messenger of Allah, there is a man asking permission to enter your house."The Messenger of Allah ﷺ said, "I think he is so-and-so," referring to Hafsa's paternal uncle through breastfeeding.Ayesha said, "If so-and-so were alive," meaning her own paternal uncle through breastfeeding, "could he enter upon me?"The Messenger of Allah ﷺ replied, "Breastfeeding makes unlawful what birth makes unlawful."
أخبرنا هارون بن عبد الله قال حدثنا معن قال حدثنا مالك عن عبد الله بن أبي بكر عن عمرة أن
أن رسول الله ﷺ كان عندها وأنها سمعت رجلا يستأذن في بيت حفصة قالت عائشة فقلت يا رسول الله هذا رجل يستأذن في بيتك فقال رسول الله ﷺ أراه فلانا لعم حفصة من الرضاعة قالت عائشة فقلت لو كان فلان حيا لعمها من الرضاعة دخل علي فقال رسول الله ﷺ إن الرضاعة تحرم ما يحرم من الولادة
Bukhari narrated it, no. 2646, see the previous hadith, and Muslim, Radaah, chapter, What is forbidden by breastfeeding is what is forbidden by birth, no. 1444, from the hadith of Malik with the same wording, and it is in Muwatta (Yahya) 2/601, and Kubra, no. 5470.