Sahih Bukhari Volume 6, Book 60, Hadith Number 204.

Narrated By Muhammad bin Abbas bin Ja’far : 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.”