Musnad Abu Ya'la al-Mawsili 109:140

Ibrahim narrated to us, Abd al-Rahman ibn Mahdi narrated to us, from Sufyan, from Salamah, who said: Someone who heard Jabir informed me, saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever sells a slave who possesses wealth, then his wealth belongs to the seller, unless the buyer stipulates [otherwise]."