Sahih Bukhari Volume 4, Book 55, Hadith Number 635.

Narated By Abu Huraira : The Prophet said, “Solomon (the son of) David said, ‘Tonight I will sleep with seventy ladies each of whom will conceive a child who will be a knight fighting for “Allah’s Cause.’ His companion said, ‘If Allah will.’ But Solomon did not say so; therefore none of those women got pregnant except one who gave birth to a half child.” The Prophet further said, “If the Prophet Solomon had said it (i.e. ‘If Allah will’) he would have begotten children who would have fought in Allah’s Cause.” Shuaib and Ibn Abi Az-Zinad said, “Ninety (women) is more correct (than seventy).”