Ata (may Allah have mercy on him) said: "If the enemy takes away the weapons, slaves, or wealth of the Muslims, then Allah grants victory to the Muslims, and a man gives testimony regarding his property, then if it is found before distribution, it should be returned to him. If it has already been distributed, then it becomes part of the spoils of war."
Hadith Referenceسنن سعید بن منصور / كتاب الجهاد / 3974