Sahih Bukhari Volume 7, Book 62, Hadith Number 94.

Narrated By Ibn Abbas : The Prophet said, “If anyone of you, when having sexual intercourse with his wife, says: Bismillah, Allahumma jannibni-Sh-Shaitan wa jannib-ish-Shaitan ma razaqtana, and if it is destined that they should have a child, then Satan will never be able to harm him.”