"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)
Reference
Sahih al-Bukhari 4682
In-book reference
Book 65, Hadith 204
USC-MSA web (English)
Vol. 6, Book 60, Hadith 204
Arabic 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 قَالَ كَانَ الرَّجُلُ يُجَامِعُ امْرَأَتَهُ فَيَسْتَحِي أَوْ يَتَخَلَّى فَيَسْتَحِي فَنَزَلَتْ {أَلاَ إِنَّهُمْ يَثْنُونَ صُدُورَهُمْ}