Musannaf Ibn Abi Shayba 13:2565

Abu Bakr narrated to us, he said: 'Abdah bin Sulayman narrated to us, from Ubaydullah, from Nafi', from Ibn 'Umar, who said: 'Umar said: "Whoever sells a slave who possesses wealth, then his wealth belongs to his master, unless the one who bought him stipulates [otherwise]."