Musannaf Ibn Abi Shayba 20:27

Abu Bakr narrated to us, he said: Hatim ibn Isma'il narrated to us, from Ja'far, from his father, who said: Ali said: "Whoever sells a slave who possesses wealth, then his wealth belongs to the seller, unless the buyer stipulates [otherwise]." The Messenger of Allah, may Allah bless him and grant him peace, decreed that.