Sahih Bukhari Volume 9, Book 93, Hadith Number 493.

Narrated By Ibn ‘Abbas : Allah’s Apostle said, “If anyone of you, when intending to have a sexual relation (sleep) with his wife, says: Bismillah, Allahumma jannibna ashShaitan, wa Jannib ash-Shaitana ma razaqtana, Satan would never harm that child, should it be ordained that they will have one. (Because of that sleep).”