Musnad Ahmad 31:576

Umar ibn Ubayd narrated to us, from Abu Ishaq, from Abu al-Wadak, from Abu Sa’id, who said: The Messenger of Allah, peace and blessings of Allah be upon him, was asked about *al-‘azl* (coitus interruptus). He said: "Not from all of the fluid is the child created. And when Allah wills to create something from it, nothing can prevent it."