Sahih Al Bukhari – Book of Invocations

Volumn 008, Book 075, Hadith Number 397.
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Narated By Ibn 'Abbas : The Prophet said, "If anyone of you, when intending to have a sexual intercourse with his wife, says: 'Bismillah, Allahumma jannibna-sh-shaitan, wa jannibi-sh-shaitan ma razaqtana,' and if the couple are destined to have a child (out of that very sexual relation), then Satan will never be able to harm that child."