Musnad Ahmad 30:315

Abu Mu'awiyah narrated to us, Al-A'mash narrated to us, from Abu Salih, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The spoils of war were not made lawful for a people with black heads before you; fire used to descend from the sky and consume them." Then, when the day of Badr arrived, the people hastened toward the spoils of war. So Allah, the Almighty and Majestic, revealed: "Had it not been for a prior decree from Allah, a great punishment would have touched you for what you took. So consume what you have taken of war booty as lawful and good." [Al-Anfal: 69]