Musnad Ahmad 32:1825

'Abdullah ibn Bakr narrated to us, he said: Humayd narrated to me, from Anas ibn Malik, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) held a wedding feast when he consummated his marriage with Zaynab bint Jahsh, and he fed the people until they were full with bread and meat. Then he went out to the chambers of the Mothers of the Believers, as he used to do on the morning of his consummation, and he would greet them and pray for them, and they would greet him and pray for him. When he returned to his house, he saw two men who were engaged in conversation. When he saw them, he turned back from his house. When the two men saw the Prophet of Allah (peace and blessings of Allah be upon him) turn back from his house, they jumped up quickly." He said: "I do not know whether I informed him of their departure or he was informed. So he returned until he entered the house, and he let down the curtain between me and him, and the verse of the Hijab was revealed."