Sahih Muslim Book 15, Hadith Number 4070.

Chapter : Expressing the words “God willing” in the oath.

Abu Huraira reported Allah’s Apostle (may peace be upon him) as saying that Sulaiman b. Dawud (once) said: 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). This hadith has been narrated on the authority of Abu Zinad with the same chain of transmitters with a variation of (these words): “Every one of them giving birth to a child, who would have fought in the cause of Allah.”