Sahih Bukhari Volume 004, Book 054, Hadith Number 503.

Narated By Ibn 'Abbas : The Prophet said, "If anyone of you, on having sexual relation with his wife, says: 'O Allah! Protect me from Satan, and prevent Satan from approaching the offspring you are going to give me,' and if it happens that the lady conceives a child, Satan will neither harm it nor be given power over it."

Related Hadith(s)