Sahih al-Bukhari 4681
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:
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 فَقَالَ أُنَاسٌ كَانُوا يَسْتَحْيُونَ أَنْ يَتَخَلَّوْا فَيُفْضُوا إِلَى السَّمَاءِ، وَأَنْ يُجَامِعُوا نِسَاءَهُمْ فَيُفْضُوا إِلَى السَّمَاءِ، فَنَزَلَ ذَلِكَ فِيهِمْ.
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."