Sahih Bukhari Volume 6, Book 65, Hadith 4684

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}‏

Narrated By Muhammad bin Abbad 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."