"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 4681
Arabic hadith text
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 فَقَالَأُنَاسٌ كَانُوا يَسْتَحْيُونَ أَنْ يَتَخَلَّوْا فَيُفْضُوا إِلَى السَّمَاءِ، وَأَنْ يُجَامِعُوا نِسَاءَهُمْ فَيُفْضُوا إِلَى السَّمَاءِ، فَنَزَلَ ذَلِكَ فِيهِمْ.
Translation
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."