Musannaf Ibn Abi Shayba 13:1911

Abu Bakr narrated to us, he said: Waki' narrated to us, he said: Sufyan narrated to us, from Khalid, from Abu Qilabah, that the Prophet, may Allah bless him and grant him peace, said: "The *mudabbar* (a slave promised freedom upon the master's death) is from the one-third."