Sahih Bukhari
Ablutions (Wudu)
Volume: 001 / Book Number: 004 / Hadith Number: 143
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."