Musnad Abu Ya'la al-Mawsili 108:210

Amr narrated to us, Sufyan narrated to us, from Amr, from Jabir, who said: A man made a slave of his *mudabbar* (stipulated to be free after his master's death), and he had no wealth other than him. So the Prophet, peace and blessings of Allah be upon him, sold him, and Nu'aym ibn al-Nahham bought him from him.