Narrated Anas:
When Allah's Messenger (ﷺ) married Zainab bint Jahsh, he made the people eat meat and bread to their fill (by giving a Walima banquet). Then he went out to the dwelling places of the mothers of the believers (his wives), as he used to do in the morning of his marriage. He would greet them and invoke good on them, and they (too) would return his greeting and invoke good on him. When he returned to his house, he found two men talking to each other; and when he saw them, he went out of his house again. When those two men saw Allah's Messenger (ﷺ): going out of his house, they quickly got up (and departed). I do not remember whether I informed him of their departure, or he was informed (by somebody else). So he returned, and when he entered the house, he lowered the curtain between me and him. Then the Verse of Al-Hijab was revealed.
USC-MSA web (English) reference: Vol. 6, Book 60, Hadith 317
Arabic reference: Book 65, Hadith 4794