Musnad Ahmad 31:697

'Affan narrated to us, Wuhayb narrated to us, Musa ibn 'Uqbah narrated to us, he said: Muhammad ibn Yahya ibn Habban narrated to me, from Ibn Muhayriz, from Abu Sa'id al-Khudri, regarding the expedition of Banu al-Mustaliq, that they captured female captives, and they wanted to practice coitus interruptus (*'azl*) with them, but not cause them to conceive. So they asked the Messenger of Allah, peace and blessings of Allah be upon him, and he said: "It does not matter if you do not do so, for Allah, the Mighty and Majestic, has already written whoever is to be the creator [of a soul] until the Day of Resurrection."