Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrates from the Prophet (peace and blessings be upon him): "When one of you intends to have intercourse with his wife, let him say: 'Bismillah, Allahumma jannibnish-shaytan, wa jannibish-shaytana ma razaqtana' (In the name of Allah. O Allah, keep Satan away from me and from the child You grant me). Then whatever child is granted, or decreed between them, Satan will not harm it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13844