Sahih al-Bukhari 4681
Narrated Muhammad bin `Abbas bin Ja`far:
That he heard Ibn `Abbas reciting: "No doubt! They fold up their breasts." (11.5) and asked him 
about its explanation. He said, "Some people used to hide themselves while answering the call of 
nature in an open space lest they be exposed to the sky, and also when they had sexual relation with 
their wives in an open space lest they be exposed to the sky, so the above revelation was sent down 
regarding them."
حَدَّثَنَا الْحَسَنُ بْنُ مُحَمَّدِ بْنِ صَبَّاحٍ، حَدَّثَنَا حَجَّاجٌ، قَالَ قَالَ ابْنُ جُرَيْجٍ أَخْبَرَنِي مُحَمَّدُ بْنُ عَبَّادِ بْنِ جَعْفَرٍ، أَنَّهُ سَمِعَ ابْنَ عَبَّاسٍ، يَقْرَأُ ‏{‏أَلاَ إِنَّهُمْ تَثْنَوْنِي صُدُورُهُمْ‏}‏ قَالَ سَأَلْتُهُ عَنْهَا فَقَالَ أُنَاسٌ كَانُوا يَسْتَحْيُونَ أَنْ يَتَخَلَّوْا فَيُفْضُوا إِلَى السَّمَاءِ، وَأَنْ يُجَامِعُوا نِسَاءَهُمْ فَيُفْضُوا إِلَى السَّمَاءِ، فَنَزَلَ ذَلِكَ فِيهِمْ‏.‏