Musannaf Ibn Abi Shayba 13:2567

Abu Bakr narrated to us, he said: Ubaydullah narrated to us, from Hanzala, from Tawus, that he was asked about a man who bought a slave and stipulated [that the slave's] wealth [belongs to the slave]. He said: "His wealth is his. And if he did not stipulate [it], then his wealth is for his master."