Musannaf Ibn Abi Shayba 13:1400

Abu Bakr narrated to us, he said: Ibn al-Mubarak narrated to us, from Abu Hamza al-Sukkari, from Yazid al-Nahwi, from Mujahid, from Abu Hurayrah, who said: A woman from Quraysh made a slave of hers a *mudabbar* (stipulated to be free after her death), then she wanted to enter into a *kitabah* (contract of manumission) with him. So the Messenger (ﷺ) wrote to Abu Hurayrah, saying: "Enter into a *kitabah* with him; if he fulfills his *kitabah*, then that is it, and if an event befalls him (i.e., he dies), he is free." He said: And I think he meant whatever remained upon him of his *kitabah* is nothing.