Sahih al-Bukhari 4682
Sahih al-Bukhari · Book 65: Prophetic Commentary on the Qur'an (Tafseer of the Prophet (pbuh)) · "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)
Narrated Muhammad bin `Abbas bin Ja`far:
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}
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."