Sahih Bukhari Volume 008, Book 074, Hadith Number 255.
Narated By Anas bin Malik: That he was a boy of ten at the time when the Prophet emigrated to Medina. He added: I served Allah's Apostle for ten years (the last part of his life time) and I know more than the people about the occasion whereupon the order of Al-Hijab was revealed (to the Prophet). Ubai b n Ka'b used to ask me about it. It was revealed (for the first time) during the marriage of Allah's Apostle with Zainab bint Jahsh. In the morning, the Prophet was a bride-groom of her and he Invited the people, who took their meals and went away, but a group of them remained with Allah's Apostle and they prolonged their stay. Allah's Apostle got up and went out, and I too, went out along with him till he came to the lintel of 'Aisha's dwelling place. Allah's Apostle thought that those people had left by then, so he returned, and I too, returned with him till he entered upon Zainab and found that they were still sitting there and had not yet gone. The Prophet went out again, and so did I with him till he reached the lintel of 'Aisha's dwelling place, and then he thought that those people must have left by then, so he returned, and so did I with him, and found those people had gone. At that time the Divine Verse of Al-Hijab was revealed, and the Prophet set a screen between me and him (his family).