Musannaf Ibn Abi Shayba 28:995

Jarir narrated to us, from Mughira, from Ibrahim, who said: If the enemy captures a man from the Muslims and a merchant buys him, he (the captive) shall work for the merchant until he pays off to him what he bought him for. And if they capture a slave belonging to the Muslims and a merchant buys him, then his master finds him, he (the master) has more right to him for his price. And if they buy a man from the people of the Dhimma, he shall work for the merchant until he pays off to him his price.