Musnad Ahmad 31:877

Abu Kamil narrated to us, Ibrahim ibn Sa'd narrated to us, Ibn Shihab narrated to us, from Ubayd Allah ibn Utbah, from Abu Sa'id al-Khudri, who said: The Prophet, peace and blessings of Allah be upon him, was asked about *al-'azl* (coitus interruptus). He said: "If you do that, there is no harm upon you if you do not do it, for there is no soul that Allah has decreed to exist but it will exist."