From Sahih Bukhari (Hadith 2819):
Narrated Abu Huraira: Allah's Messenger (ﷺ) said, "Once Solomon, son of David said, '(By Allah) Tonight I will have sexual intercourse with one hundred (or ninety-nine) women each of whom will give birth to a knight who will fight in Allah's Cause.' On that, a companion (angel) said to him, "(Say) Allah willing", but he did not say, "Allah willing." Therefore only one of those women conceived and gave birth to a half-man. By Him in Whose Hands Muhammad's life is, if he had said, "Allah willing', (he would have begotten sons) all of whom would have been knights striving in Allah's Cause
Reference: Bukhari 2819