Amrah narrates from Aishah (may Allah be pleased with her) that the Prophet (peace and blessings be upon him) was with her, and she heard the voice of a man asking permission to enter the house of Hafsah (may Allah be pleased with her). Aishah (may Allah be pleased with her) said: I said: "O Messenger of Allah, this man is asking permission to enter your house." The Messenger of Allah (peace and blessings be upon him) said: "I know him; he is the foster uncle of Hafsah (may Allah be pleased with her)." I said: "O Messenger of Allah, if so-and-so, that is, her foster uncle, were alive, could he come in to me?" The Messenger of Allah (peace and blessings be upon him) said: "Yes! Suckling makes unlawful the relations that birth makes unlawful."
These are the words of Imam al-Shafi'i's hadith. Imam Bukhari recorded it in the Sahih from Isma'il bin Abi Uways and other narrators, and Imam Muslim narrated it from Yahya bin Yahya.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرضاع / 15605