Narrated Ibn 'Abbas:

The Prophet said, "If anyone of you, when intending to have a sexual intercourse with his wife, says: 'Bismillah, Allahumma jannibna-sh-shaitan, wa jannibi-sh-shaitan ma razaqtana,' and if the couple are destined to have a child (out of that very sexual relation), then Satan will never be able to harm that child."