Narrated Anas bin Malik:

that he was a boy of ten at the time when the Prophet emigrated to Medina. He added: I served Allah's Apostle for ten years (the last part of his life time) and I know more than the people about the occasion whereupon the order of Al-Hijab was revealed (to the Prophet). Ubai b n Ka'b used to ask me about it. It was revealed (for the first time) during the marriage of Allah's Apostle with Zainab bint Jahsh. In the morning, the Prophet was a bride-groom of her and he Invited the people, who took their meals and went away, but a group of them remained with Allah's Apostle and they prolonged their stay. Allah's Apostle got up and went out, and I too, went out along with him till he came to the lintel of 'Aisha's dwelling place. Allah's Apostle thought that those people had left by then, so he returned, and I too, returned with him till he entered upon Zainab and found that they were still sitting there and had not yet gone. The Prophet went out again, and so did I with him till he reached the lintel of 'Aisha's dwelling place, and then he thought that those people must have left by then, so he returned, and so did I with him, and found those people had gone. At that time the Divine Verse of Al-Hijab was revealed, and the Prophet set a screen between me and him (his family).

حَدَّثَنَا يَحْيَى بْنُ سُلَيْمَانَ، حَدَّثَنَا ابْنُ وَهْبٍ، أَخْبَرَنِي يُونُسُ، عَنِ ابْنِ شِهَابٍ، قَالَ أَخْبَرَنِي أَنَسُ بْنُ مَالِكٍ، أَنَّهُ كَانَ ابْنَ عَشْرِ سِنِينَ مَقْدَمَ رَسُولِ اللَّهِ ﷺ الْمَدِينَةَ، فَخَدَمْتُ رَسُولَ اللَّهِ ﷺ عَشْرًا حَيَاتَهُ، وَكُنْتُ أَعْلَمَ النَّاسِ بِشَأْنِ الْحِجَابِ حِينَ أُنْزِلَ، وَقَدْ كَانَ أُبَىُّ بْنُ كَعْبٍ يَسْأَلُنِي عَنْهُ، وَكَانَ أَوَّلَ مَا نَزَلَ فِي مُبْتَنَى رَسُولِ اللَّهِ ﷺ بِزَيْنَبَ ابْنَةِ جَحْشٍ، أَصْبَحَ النَّبِيُّ ﷺ بِهَا عَرُوسًا فَدَعَا الْقَوْمَ، فَأَصَابُوا مِنَ الطَّعَامِ ثُمَّ خَرَجُوا، وَبَقِيَ مِنْهُمْ رَهْطٌ عِنْدَ رَسُولِ اللَّهِ ﷺ فَأَطَالُوا الْمُكْثَ فَقَامَ رَسُولُ اللَّهِ ﷺ فَخَرَجَ وَخَرَجْتُ مَعَهُ كَىْ يَخْرُجُوا، فَمَشَى رَسُولُ اللَّهِ ﷺ وَمَشَيْتُ مَعَهُ حَتَّى جَاءَ عَتَبَةَ حُجْرَةِ عَائِشَةَ، ثُمَّ ظَنَّ رَسُولُ اللَّهِ ﷺ أَنَّهُمْ خَرَجُوا فَرَجَعَ وَرَجَعْتُ مَعَهُ، حَتَّى دَخَلَ عَلَى زَيْنَبَ فَإِذَا هُمْ جُلُوسٌ لَمْ يَتَفَرَّقُوا، فَرَجَعَ رَسُولُ اللَّهِ ﷺ وَرَجَعْتُ مَعَهُ، حَتَّى بَلَغَ عَتَبَةَ حُجْرَةِ عَائِشَةَ، فَظَنَّ أَنْ قَدْ خَرَجُوا، فَرَجَعَ وَرَجَعْتُ مَعَهُ، فَإِذَا هُمْ قَدْ خَرَجُوا، فَأُنْزِلَ آيَةُ الْحِجَابِ، فَضَرَبَ بَيْنِي وَبَيْنَهُ سِتْرًا.