Musnad Ahmad 32:480

Abu 'Asim narrated to us, Abu 'Amr Mubarak al-Khayyat, the grandfather of the children of 'Abbad bin Kathir, informed us, saying: I asked Thumamah bin 'Abdullah bin Anas about *al-'azl* (coitus interruptus). He said: I heard Anas bin Malik saying: A man came to the Messenger of Allah, may Allah bless him and grant him peace, and asked about *al-'azl*. The Messenger of Allah, may Allah bless him and grant him peace, said: "If the fluid from which the child is created were poured onto a rock, Allah would bring forth from it—or 'He brings forth from it' (the doubt is from him)—a child, and Allah will certainly create a soul that He is the Creator of."