Sahih Muslim Book 8, Hadith Number 3399.

‘Aisha (Allah be pleased with her) reported that there came Aflah the brother, of Abu’l-Qu’ais, who sought her permission (to enter) after seclusion was instituted, and Abu-Qu’ais was the father of ‘Aisha by reason of fosterage. ‘Aisha said: By Allah, I would not permit Aflah unless I have solicited the opinion of Allah’s Messenger (May peace be upon him) for Abu Qulais has not suckled me, but his wife has given me suck. ‘Aisha'(Allah be pleased with her) said: When Allah’s Messenger (May peace be upon him) entered, I said: Allah’s Messenger, Aflah is the brother of Abu’l-Qulais; he came to me to seek my permission for entering (the house). I did not like the idea of granting him permission until I had solicited your opinion. Thereupon Allah’s Apostle (May peace be upon him) said: Grant him permission. ‘Urwa said it was on account of this that ‘Aisha used to say. What is unlawful by reason of consanguinity is unlawful by reason of fosterage.