Sahih al-Bukhari 4682
Narrated Muhammad bin `Abbas bin Ja`far:
Ibn `Abbas recited. "No doubt! They fold up their breasts." I said, "O Abu `Abbas! What is meant by 
"They fold up their breasts?" He said, "A man used to feel shy on having sexual relation with his wife 
or on answering the call of nature (in an open space) so this Verse was revealed:-- "No doubt! They 
fold up their breasts."
حَدَّثَنِي إِبْرَاهِيمُ بْنُ مُوسَى، أَخْبَرَنَا هِشَامٌ، عَنِ ابْنِ جُرَيْجٍ، وَأَخْبَرَنِي مُحَمَّدُ بْنُ عَبَّادِ بْنِ جَعْفَرٍ، أَنَّ ابْنَ عَبَّاسٍ، قَرَأَ ‏{‏أَلاَ إِنَّهُمْ تَثْنَوْنِي صُدُورُهُمْ‏}‏ قُلْتُ يَا أَبَا الْعَبَّاسِ مَا تَثْنَوْنِي صُدُورُهُمْ قَالَ كَانَ الرَّجُلُ يُجَامِعُ امْرَأَتَهُ فَيَسْتَحِي أَوْ يَتَخَلَّى فَيَسْتَحِي فَنَزَلَتْ ‏{‏أَلاَ إِنَّهُمْ يَثْنُونَ صُدُورَهُمْ‏}‏