The Messenger of Allah (peace be upon him) said: "If one of you marries a woman or buys a slave, he should say: "O Allah, I ask You [to benefit me of] her goodness and the goodness for which she was created; I take refuge in You from her evil and the evil for which she created. Then he [the husband] should take hold of her forehead and invoke Allah for blessing." (1)
The Prophet (peace be upon him) 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." (2)
The meaning of the Hadith:
Al Qurtuby said: The meaning of the Hadith is that a baby who is born after his parents say these invocations shall be protected from the mislead of Satan, and Satan shall have no power over him because he or she shall be among the protected servants of Allah who are mentioned in Allah's Saying: "Verily, My slaves (i.e. the true believers of Islâmic Monotheism) — you have no authority over them. And All-Sufficient is your Lord as a Guardian." (3)
This protection is because of the piety of the righteous parents, the blessing of Allah's Name, and seeking refuge in Allah and taking shelter in him; as if this is part of the saying of Mary: "And I seek refuge with You (Allâh) for her and for her offspring from Shaitân (Satan), the outcast."(4)
This does not mean that Satan will not insinuate him or incite rivalries between him and the people; all these may happen, but Allah (may He be Exalted) preserves that child from harm whether in his heart, his religion, and all his matters. And Allah knows best. (5)
Ibn Hajar after he had mentioned several meanings said: Many people who know the virtue of this great invocation forget to say it when they want to have sexual intercourse with their wives, but few who remember to say it, nevertheless pregnancy does not take place. (6)
(1) Reported by Abu Dawud in the book of marriage, chapter on general rulings on marriage, Hadith No. 2160 - 2/255, Ibn Majah in the book of marriage, chapter on what a husband should say when his wife comes in to him, Hadith No. 1918 - 1/617, Al Bayhaqy in the book of marriage, chapter on what should a husband say when he wants to have sexual intercourse with his wife, Hadith No. 7/148, and Al Hakim in the book of marriage, chapter on invocation of a person who wants to have sexual intercourse with his slave girl or wife, 2/185 and said: This is an authentic Hadith according to the narration of the trustworthy narrators on the authority of `Amr ibn Shu`ayb and Adh-Dhahaby agreed to his judgment on the Hadith. The Hadith is good according to the judgment of Al `Iraqy in Takhrij Ahadith Al Ihya' 1/298 and Al Albany in the etiquettes of marriage p. 21, and Sharh As-Sunnah of Al Baghawy 5/118.
(2) Reported by Al Bukhari in several positions of which is the book of the beginning of creation, chapter on the attributes of Satan and its troops, Hadith No. 3271 - 6/335 with Fathul-Bary and reported by Muslim in the book of marriage, chapter on what should be said at having a sexual intercourse 10/5 with the explanation of An-Nawawy.