Sahih Muslim Book 8, Hadith Number 3371.

Abu Sirma said to Abu Sa’id al Khadri (Allah he pleased with him): O Abu Sa’id, did you hear Allah’s Messenger (May peace be upon him) mentioning al-‘azl? He said: Yes, and added: We went out with Allah’s Messenger (May peace be upon him) on the expedition to the Bi’l-Mustaliq and took captive some excellent Arab women; and we desired them, for we were suffering from the absence of our wives, (but at the same time) we also desired ransom for them. So we decided to have sexual intercourse with them but by observing ‘azl (Withdrawing the male sexual organ before emission of semen to avoid conception). But we said: We are doing an act whereas Allah’s Messenger is amongst us; why not ask him? So we asked Allah’s Messenger (May peace be upon him), and he said: It does not matter if you do not do it, for every soul that is to be born up to the Day of Resurrection will be born.