Sahih Muslim Book 8, Hadith Number 3336.

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 (Allah be pleased with him) reported: When Allah’s Apostle (May peace be upon him) contracted marriage with Zainab (Allah be pleased with bet), Umm Sulaim sent him hats in a vessel of stone as a gift. Anas stated that Allah’s Messenger (May peace be upon him) said to him: Go and invite on my behalf all the Muslims whom you meet. So I invited on his behalf everyone whom I met. They entered (his house) and they ate and went out. And Allah’s Messenger (May peace be upon him) had kept his hand on the food, and he invoked blessing on that, and said whatever Allah wished him to say, and none whom I met was left uninvited. They ate to their fill and went out, but a group among them remained there and was engaged in lengthy discussion. Allah’s Apostle (May peace be upon him) felt shy of saying them anything. So he went out and left them in his house and Allah the Great 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.” Qatada (instead of using the word Ghaira Nazirina) used the word Ghaira Mutahayyinina (i.e. not waiting for the time of the food). But when you are invited, enter… up to this verse. This is purer for your hearts and their hearts.