Al-Lu'lu Wal-Marjan 904 — The Book of Hajj (Pilgrimage to Makka)

Narrated Anas رضي الله عنه : I know (about) the Hijâb (the order of veiling of women) more than anybody else. Ubai bin Ka'b used to ask me about it. Allâh's Messenger صلى الله عليه وسلم became the bridegroom of Zainab bint Jahsh whom he married at Al-Madīna. After the sun had risen high in the sky, the Prophet ﷺ invited the people for a meal. Allāh's Messenger ﷺ remained sitting and some people remained sitting with him after the other guests had left. Then Allāh's Messenger ﷺ got up and went away, and I, too, followed him till he reached the door of 'Āisha's room. Then he thought that the people must have left the place by then, so he returned and I, also, returned with him. Behold, the people were still sitting at their place. So he went back again for the second time, and I went along with him, too. When we reached the door of 'Āisha's room, he returned and I, also, returned with him to see that the people had left. Thereupon the Prophet ﷺ hung a curtain between me and him, and the Verse regarding the order for (veiling of women) Hijāb was revealed. ( Sahih Al-Bukhārī, Hadīth No. 375, Vol. 7 )