Musnad Abu Ya'la al-Mawsili 42:180

Abu Khaythama narrated to us, he said: Abd al-Rahman ibn Mahdi narrated to us, from Sufyan, from Abu Ishaq, from Abu al-Wadak, from Abu Sa’id, who said: We captured women on the day of Hunayn, and we used to practice withdrawal (*‘azl*) with them. Some of us said to others: "Do you do this while the Messenger of Allah (peace be upon him) is among you?" So we asked the Messenger of Allah (peace be upon him), and he said: "Not every fluid results in a child. When Allah wills to create something, nothing can prevent it."