Sayyiduna Anas bin Malik (may Allah be pleased with him) said: I am the most knowledgeable of all people about the ruling of hijab; Ubayy bin Ka'b (may Allah be pleased with him) used to ask me about it. Sayyiduna Anas (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) woke up as the bridegroom of Sayyidah Zaynab bint Jahsh (may Allah be pleased with her); he (peace and blessings be upon him) had married her in Madinah. He (peace and blessings be upon him) invited the people to a meal after the day had advanced. The Messenger of Allah (peace and blessings be upon him) remained seated, and some men stayed sitting with him (peace and blessings be upon him) after the people had left, until the Messenger of Allah (peace and blessings be upon him) got up and walked, and I walked with him (peace and blessings be upon him), until he (peace and blessings be upon him) reached the door of Sayyidah Aishah's (may Allah be pleased with her) room. Then he (peace and blessings be upon him) thought that they had gone out, so he returned and I returned with him (peace and blessings be upon him), and we found that they had only just got up. So he (peace and blessings be upon him) let down a curtain between me and himself, and the verse of hijab was revealed.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20501