Sahih Muslim Book 15, Hadith Number 4066.

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

Abu Huraira reported that (Hadrat) Sulaiman had sixty wives. He (one day) said: I will visit each one of them every night, and every one of them will become pregnant and give birth to a male child who will be a horseman and fight in the cause of Allah. But (it so happened) that none of them became pregnant except one, but she gave birth to an incomplete child. Thereupon Allah’s Messenger (may peace be upon him) said: Had he said Insha’ Allah (if God so wills), then every one of them would have given birth to a child who would have been a horseman and fought in the cause of Allah