Sahih Muslim Book 27, Hadith 35
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.
Abu Huraira reported this hadith from the Messenger of Allah (ﷺ) through another chain of transmitters.
I will go round in the night to my ninety wives, and every one of them will give birth to a child (who will grow up) as a horseman and fight in the cause of Allah His companions said to him: Say" Insha' Allah." but he did not say Inshii' Allah. He went round all of them but none of them became pregnant but one, and she gave birth to a premature child. And by Him in Whose hand is the life of Muhammad, if he had said, Insha' Allah (his wives would have given birth to the children who would all have grown up into horsemen and fought in the way of Allah).