Musannaf Ibn Abi Shayba 28:813

Waki' narrated to us, he said: 'Uqbah narrated to us, he said: I heard 'Abdullah ibn Buraydah being asked about food that is found in the land of the enemy. He said: "If he sold any of it for a dirham, he must return it; otherwise, it is *ghulul* (ill-gotten booty)."