Sahih Al Bukhari 1: Chapter 4, Hadith 141

(141) To recite “In the Name of Allah,” during every action and on having sexual relations with one's wife

حَدَّثَنَا عَلِيُّ بْنُ عَبْدِ اللَّهِ، ‏‏‏‏‏‏قَالَ:‏‏‏‏ حَدَّثَنَا جَرِيرٌ، ‏‏‏‏‏‏عَنْ مَنْصُورٍ، ‏‏‏‏‏‏عَنْ سَالِمِ بْنِ أَبِي الْجَعْدِ، ‏‏‏‏‏‏عَنْ كُرَيْبٍ، ‏‏‏‏‏‏عَنِ ابْنِ عَبَّاسٍ، ‏‏‏‏‏‏يَبْلُغُ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ، ‏‏‏‏‏‏قَالَ:‏‏‏‏ "لَوْ أَنَّ أَحَدَكُمْ إِذَا أَتَى أَهْلَهُ، ‏‏‏‏‏‏قَالَ:‏‏‏‏ بِاسْمِ اللَّهِ، ‏‏‏‏‏‏اللَّهُمَّ جَنِّبْنَا الشَّيْطَانَ، ‏‏‏‏‏‏وَجَنِّبْ الشَّيْطَانَ مَا رَزَقْتَنَا، ‏‏‏‏‏‏فَقُضِيَ بَيْنَهُمَا وَلَدٌ لَمْ يَضُرُّهُ".

SAHIH HADITH:

Narrated Ibn `Abbas: The Prophet 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.

REFERENCE:

Sahih Al Bukhari 1: Chapter 4, Hadith 141