Musnad Ahmad 32:83

Ibn Abi ‘Adi narrated to us, from Humayd, from Anas, who said: I invited the Muslims to the wedding feast of the Messenger of Allah (may Allah bless him and grant him peace) on the morning he consummated his marriage with Zaynab bint Jahsh. He fed the Muslims until they were full with bread and meat. He said: Then he returned as he used to do, and he came to the chambers of his wives and greeted them, and they prayed for him. He said: Then he returned to his house, and I was with him. When he reached the house, there were two men who had been talking to each other in a corner of the house. When he saw them, he turned back. When the two men saw that the Prophet (may Allah bless him and grant him peace) had turned away from his house, they stood up quickly. I do not know whether I informed him or he was informed, but he returned to his house and let down the curtain between me and him, and the verse of the Hijab was revealed.