Narrated Anas bin Malik:
I was ten years old when Allah's Apostle arrived at Medina. My mother and aunts used to urge me to serve the Prophet regularly, and I served him for ten years. When the Prophet died I was twenty years old, and I knew about the order of Al-Hijab (veiling of ladies) more than any other person when it was revealed. It was revealed for the first time when Allah's Apostle had consummated his marriage with Zainab bint Jahsh. When the day dawned, the Prophet was a bridegroom and he invited the people to a banquet, so they came, ate, and then all left except a few who remained with the Prophet for a long time..

The Prophet got up and went out, and I too went out with him so that those people might leave too. The Prophet proceeded and so did I, till he came to the threshold of 'Aisha's dwelling place. Then thinking that these people have left by then, he returned and so did I along with him till he entered upon Zainab and behold, they were still sitting and had not gone. So the Prophet again went away and I went away along with him. When we reached the threshold of 'Aisha's dwelling place, he thought that they had left, and so he returned and I too, returned along with him and found those people had left. Then the Prophet drew a curtain between me and him, and the Verses of Al-Hijab were revealed.

Sahih Bukhari
Volume 7, Book 62, Number 95