Musannaf Ibn Abi Shayba 28:567

Muhammad ibn Bakr narrated to us, from Ibn Jurayj, who said: I heard Nafi’ saying: We have never ceased hearing, since forever, that when the Muslims and the disbelievers meet, and a man from the Muslims kills a man from the disbelievers, then his spoils belong to him, unless it is in the heat of battle, for then it is not known who killed the slain person.