Sahih Al Bukhari – Book of Ablutions (Wudu')
Volumn 001, Book 004, Hadith Number 143.
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Narated By Ibn 'Abbas RA : The Prophet PBUH said, "If anyone of you on having sexual relations with his wife said (and he must say it before starting) 'In the name of Allah. O Allah! Protect us from Satan and also protect what you bestow upon us (i.e. the coming offspring) from Satan, and if it is destined that they should have a child then, Satan will never be able to harm that offspring."