"No doubt! They did fold up their breasts, that they may hide from Him. Surely, even when they cover themselves with their garments, He knows what they conceal and what they reveal. Verily, He is the All-Knower of the (innermost secrets) of the breasts." (V.11:5) Sahih al-Bukhari 4682
Arabic hadith text
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 قَالَكَانَ الرَّجُلُ يُجَامِعُ امْرَأَتَهُ فَيَسْتَحِي أَوْ يَتَخَلَّى فَيَسْتَحِي فَنَزَلَتْ {أَلاَ إِنَّهُمْ يَثْنُونَ صُدُورَهُمْ}
Translation
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."