Abu Sa'id al-Khudri (may Allah be pleased with him) said that he was sitting with the Prophet (peace and blessings be upon him) when a man of the Ansar came and said: "O Messenger of Allah, we have slave women, and we love (their) prices; may we practise coitus interruptus ('azl) with them?" He (peace and blessings be upon him) asked: "Do you do that?" Then he said: "It is not binding on you (to refrain); the soul that is to be born will certainly come into being, whatever you do."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21789