Musnad Ahmad 33:102

Waki’ and ‘Abd al-Rahman narrated to us, from Sufyan, from Salamah ibn Kuhayl, from the one who heard—‘Abd al-Rahman said: The one who heard Jabir ibn ‘Abd Allah narrated to 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]."