It is narrated from Aishah (may Allah be pleased with her): Aflah, the brother of Abu al-Qu'ays, asked my permission to enter, and I did not give him permission. He said: Do you veil yourself from me when I am your uncle? She said: I said: How is that? He said: The wife of my brother suckled you with my brother's milk. She said: I asked the Messenger of Allah (peace and blessings be upon him), and he said: "Aflah has spoken the truth; give him permission."
Bukhari narrated it in the Sahih from Adam, and Muslim recorded it through another chain from Shu'bah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرضاع / 15610