Sahih Al Bukhari – Book of Wedlock, Marriage (Nikaah)

Volumn 007, Book 062, Hadith Number 094.
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Narated 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."