Musnad Ahmad 31:472

'Abd al-Rahman narrated to us, from Sufyan, from Abu Ishaq, from Abu al-Wadak, from Abu Sa'id al-Khudri, who said: We captured captives on the day of Hunayn, and we began to practice withdrawal (*'azl*) with them, while we desired the ransom. Some of us said to others: "Are you doing that while the Messenger of Allah, peace and blessings of Allah be upon him, is among you?" So I asked the Messenger of Allah, peace and blessings of Allah be upon him, and he said: "Not from every fluid is the child created. When Allah wills to create something, nothing can prevent Him."