Musnad Ahmad 32:1594

Mu'ammal narrated to us, Hammad—meaning Ibn Zayd—narrated to us, Ayyub narrated to us, from Abu Qilabah, from Anas, who said: I am the most knowledgeable of people, or among the most knowledgeable of people, regarding the Verse of the Hijab. The Prophet, peace and blessings of Allah be upon him, married Zaynab bint Jahsh. He slaughtered a sheep and invited his companions. They ate and sat talking, and the Prophet, peace and blessings of Allah be upon him, began to go out and come back in while they were sitting. Then he would go out and stay for as long as Allah willed, and return while they were still sitting, and Zaynab was sitting in a corner of the house. The Prophet, peace and blessings of Allah be upon him, felt too shy to say anything to them. Then it was revealed: "O you who have believed, do not enter the houses of the Prophet except when you are permitted for a meal, without awaiting its readiness. But when you are invited, then enter," [Al-Ahzab: 53], the verses up to His saying, the Almighty and Majestic: "And when you ask [his wives] for something, ask them from behind a partition," [Al-Ahzab: 53]. He said: So the Messenger of Allah, peace and blessings of Allah be upon him, ordered the hijab to be put up in its place, and it was set up.