Sahih al-Bukhari 65:203

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."