Musnad Ahmad 32:1417

Hashim narrated to us, 'Isa ibn Tahman narrated to us, he said: I heard Anas say: Zaynab bint Jahsh used to boast over the wives of the Prophet, may Allah bless him and grant him peace, saying: "Allah married me [to him] from above the heavens." And he fed [the people] on that day bread and meat. The people were sitting just as they were in the house. Then the Messenger of Allah, may Allah bless him and grant him peace, stood up and went out. He remained for as long as Allah willed that he remain, then he returned while the people were sitting just as they were. That was difficult for him, and it was recognized upon his face, so the verse of the Hijab was revealed.