Musannaf Ibn Abi Shayba 28:823

Hushaym narrated to us, from Ibn 'Awn, from Raja' ibn Haywah, that Abu 'Ubaydah wrote to 'Umar ibn al-Khattab regarding a slave whom the polytheists had captured, then the Muslims overcame him after that. He said: "His owner has more right to him as long as he has not been divided (as spoils of war). But if he has been divided, then it is final."