Sahih Muslim Book 15, Hadith Number 4069.

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

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.