Sahih Bukhari Volume 003, Book 034, Hadith Number 432.
Narated By Abu Said Al-Khudri : That while he was sitting with Allah's Apostle he said, "O Allah's Apostle! We get female captives as our share of booty, and we are interested in their prices, what is your opinion about coitus interrupt us?" The Prophet said, "Do you really do that? It is better for you not to do it. No soul that which Allah has destined to exist, but will surely come into existence.