Musnad Ahmad 32:1534

Ya'qub narrated to us, my father narrated to us, from Salih. Ibn Shihab said: Indeed, Anas ibn Malik said: "I am the most knowledgeable of people regarding the Hijab; Ubayy ibn Ka'b used to ask me about it." Anas said: "The Messenger of Allah (ﷺ) became a bridegroom with Zaynab bint Jahsh. He had married her in Medina. He invited the people for food after the day had risen. The Messenger of Allah (ﷺ) sat, and some men sat with him after the people had stood up, until the Messenger of Allah (ﷺ) stood up and walked. I walked with him until he reached the chamber of Aisha. Then he thought that they had left, so he returned, and I returned with him. He said: 'So behold, they were sitting in their place.' So he returned, and I returned with him a second time until he reached the chamber of Aisha. He returned, and I returned with him. Behold, they had stood up. So he struck a curtain between me and him, and the Hijab was revealed."