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.”

{Volume 8, Book 75, Number 397 : Sahih Bukhari

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Abu Sa'id Al-Khudri, may Allah be pleased with him, reported:
We went out with Allah's Messenger (may peace be upon him) on the expedition to the Banu Al-Mustaliq and took captive some excellent Arab women. We were suffering from the absence of our wives, (but at the same time) we refrained from copulation with our female captives lest should we lose the right of asking ransom for them (in case they bear children for us). So we decided to have sexual intercourse with them but by observing 'azl (withdrawing the male sexual organ before emission of semen to avoid conception). But we said: We are doing an act whereas Allah's Messenger is among us; why not ask him? So we asked Allah's Messenger (may peace be upon him), and he said: It does not matter whether you perform `azl or not, for every soul that is to be born up to the Day of Resurrection will be born.