The Book of Marriage (Kitab Al-Nikah)
Muslim :: Book 8 : Hadith 3380
Abu Sa'id al-Khudri (Allah be pleased with him) reported: Mention was made about al-'azl in the presence of Allah's Messenger (may peace be upon him), whereupon he said: Why any one of you practises it? (He did not say: One of you should not do it), for there is no created soul, whose creator is not Allah.