Language:
كتاب الوضوء
4
Ablutions (Wudu')
(8)
Chapter: To recite "In the name of Allah," during every action and on having sexual relations with one's wife
(8)
باب التَّسْمِيَةِ عَلَى كُلِّ حَالٍ وَعِنْدَ الْوِقَاعِ

Narrated Ibn `Abbas:

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."

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 ‏"‏‏.‏
Reference : Sahih al-Bukhari 141
In-book reference : Book 4, Hadith 7
USC-MSA web (English) reference : Vol. 1, Book 4, Hadith 143
  (deprecated numbering scheme)