Musnad Ahmad 31:883

'Umar ibn 'Ubayd narrated to us, from Abu Ishaq, from Abu al-Wadak, from Abu Sa'id al-Khudri, who said: The Messenger of Allah (may Allah bless him and grant him peace) was asked about *al-'azl* (coitus interruptus). He said: "Not from all the fluid is the child created. When Allah wills to create something, nothing can prevent Him."