Sahih Muslim Book 27, Hadith 34
Abu Huraira reported this hadith from the Messenger of Allah (ﷺ) through another chain of transmitters.
I will have an intercourse with seventy wives during the night; all of them will give birth to a male child who will fight in the cause of Allah. His companion or the ang I said to him: Say," If God wills." But he (Hadrat Sulaimin) did not say so, and he forgot it. And none of his wives gave birth to a child, but one who gave birth to a premature child. Allah's Messenger (ﷺ) said: Had he said Insha' Allah (if God so will). he would not have failed and his desire would have been materialised.
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 (ﷺ) said: If he had said" Insha' Allah." he would not have failed, and his desire must have been fulfilled.