Musannaf Ibn Abi Shayba 35:20

Ibn ‘Uyaynah narrated to us, from ‘Amr, who heard Jabir saying: A man from the Ansar made a slave of his a *mudabbar* (stipulated to be free after his master's death), and he had no wealth other than him. So the Prophet, may Allah bless him and grant him peace, sold him. Nu‘aym, a Coptic slave, bought him. He died last year during the governorship of Ibn al-Zubayr.