Musnad Ahmad 31:778

Abu Nu'aym narrated to us, Yunus narrated to us, Abu al-Waddak Jabr ibn Nawf narrated to me, he said: Abu Sa'id narrated to me, he said: We captured female captives on the day of Hunayn, and we were practicing *‘azl* (coitus interruptus) with them, seeking to ransom them from their families. Some of us said to others: "Do you do this while the Messenger of Allah, may Allah bless him and grant him peace, is among you? Go to him and ask him." So we went to him, or we mentioned that to him, and he said: "Not from all the fluid does the child come. When Allah decrees a matter, it will be." And we passed by cooking pots, and they were boiling, so he said to us: "What is this meat?" We said: "Meat of donkeys." He said to us: "Domestic or wild?" We said to him: "Rather, domestic." He said: "Then overturn them." He said: So we overturned them, while we were hungry and craving it. He said: "And we used to be commanded to tie the mouths of the water skins."