Sahih al-Bukhari 141
Sahih al-Bukhari · Book 4: Ablutions (Wudu') · To recite "In the name of Allah," during every action and on having sexual relations with one's wife
Narrated Ibn `Abbas:
حَدَّثَنَا عَلِيُّ بْنُ عَبْدِ اللَّهِ ، قَالَ حَدَّثَنَا جَرِيرٌ ، عَنْ مَنْصُورٍ ، عَنْ سَالِمِ بْنِ أَبِي الْجَعْدِ ، عَنْ كُرَيْبٍ ، عَنِ ابْنِ عَبَّاسٍ ، يَبْلُغُ بِهِ النَّبِيَّ صلى الله عليه وسلم قَالَ " لَوْ أَنَّ أَحَدَكُمْ إِذَا أَتَى أَهْلَهُ قَالَ بِسْمِ اللَّهِ اللَّهُمَّ جَنِّبْنَا الشَّيْطَانَ وَجَنِّبِ الشَّيْطَانَ مَا رَزَقْتَنَا. فَقُضِيَ بَيْنَهُمَا وَلَدٌ، لَمْ يَضُرَّهُ " .
The Prophet (ﷺ) 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."