Sahih Muslim #3384

I was the best informed among the people pertaining to Hijab (veil and seclusion). Ubayy b. Ka'b used to ask me about it. Anas (Allah be pleased with him) thus narrated: The Messenger of Allah (ﷺ) got up in the morning as a bridegroom of Zainab bint jahsh (Allah be pleased witt her) as he had married her at Medina. He invited people to the wedding feast after the day had well risen. There sat Allah's Messenger (ﷺ) and there kept sitting along with him some persons after the people had stood up (for departure) ; then Allah's Messenger (ﷺ) stood up and walked on and I also walked along with him until he reached the door of the apartment of 'A'isha (Allah be pleased with her). He then thought that they (those who had been sitting there after meal) had gone away. So he returned and I also returned with him, but they were still sitting at their places. So he returned for the second time and I also returned until he reached the apartment of 'A'isha. He again returned and I also returned and they had (by that time) stood up, and he hung a curtain between me and him (at the door of the apartment of Hadrat Zainab, where he had to stay), and Allah revealed the verse pertaining to veil.