Intention of having relation with wife
بِسْمِ اللَّهِّ اللَّهُمَّ جَنِّبْنَا الشَّيْطَانَ وَجَنِّبِ الشَّيْطَانَ مَا رَزَقْتَنَا
"I am having relations in the name of Allah. O Allah save us from Satan and keep him away from the children You grant us". The child that is born after the recitation of this dua will never be harmed by Satan.
Note: This dua must be read for if Allah's name is not taken, the Satan's sperm enters with the man's (As in marginal notes of Hisnul Hasin)