Musnad Ahmad 31:448

Waki’ narrated to us, from Yunus ibn ‘Amr, from Abu al-Waddak, from Abu Sa’id, who said: We captured captives on the day of Hunayn, and we were seeking their ransom. We asked the Messenger of Allah, peace and blessings of Allah be upon him, about *‘azl* (coitus interruptus). He said: "Do whatever seems good to you, for whatever Allah has decreed, it will come to pass; and it is not from every drop [of fluid] that a child is created."