Sahih Bukhari: Hadith 5936

Allah, the brother of Abu Al-Qu'ais asked my permission to enter after the verses of Al-Hijab (veiling the ladies) was revealed, and I said, "By Allah, I will not admit him unless I take permission of Allah's Apostle for it was not the brother of Al-Qu'ais who had suckled me, but it was the wife of Al-Qu'ais, who had suckled me." Then Allah's Messenger (ﷺ) entered upon me, and I said, "O Allah's Messenger (ﷺ)! The man has not nursed me but his wife has nursed me." He said, "Admit him because he is your uncle (not from blood relation, but because you have been nursed by his wife), Taribat Yaminuki." `Urwa said, "Because of this reason, ' Aisha used to say: Foster suckling relations render all those things (marriages etc.) illegal which are illegal because of the corresponding blood relations." (See Hadith No. 36, Vol. 7)