Sahih Muslim Book 8, Hadith Number 3333.

Chapter: Pertaining to the marriage of Zainab bint Jahsh with the Holy Prophet (May peace be upon him) and verses pertaining to purdah veil).

Anas b. Malik (Allah be pleased with him) reported: When Allah’s Apostle (May peace be upon him) married Zainab bint jahsh, he invited people (to the wedding feast) and they ate food. They then sat there and entered into conversation. He (the Holy Prophet) made a stir as if he was preparing to stand up, but (the persons busy in talking) did not stand up. When he (the Holy Prophet) saw it, he stood up and when he did so, some other persons stood up. ‘Asim and Abd al-A’la in their narrations made this addition: Three (persons) sat there, and Allah’s Apostle (May peace be upon him) came there to enter (the apartment) but he found the people sitting there. Then they stood up and went away. He said: Then I came and informed Allah’s Apostle (May peace be upon him) that they had gone away. He (the Holy Prophet) then came there until he entered (the apartment). I also went and was about to enter, when he hung a curtain between me and him (and it was on this occasion that) Allah, the Exalted and Majestic, revealed this verse: “O you who believe, enter not the houses of the Prophet unless permission is given to you for a meal, not waiting for its cooking being finished to the (words) “Surely this is serious in the sight of Allah” (xxxiii. 53).