The Book of Oaths (Kitab Al-Aiman)
Muslim :: Book 15 : Hadith 4069
Abu Huraira reported that Sulaiman b. Dawud said: I will certainly have intercourse with seventy wives during the night, and every wife amongst them will give birth to a child, who will fight in the cause of Allah. It was said to him: Say:" Insha' Allah" (God willing), but he did not say so and forgot it. He went round them but none of them give birth to a child except one woman and that too was an incomplete person. Upon this Allah's Messenger (may peace be upon him) said: If he had said" Insha' Allah." he would not have failed, and his desire must have been fulfilled.
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