Musannaf Ibn Abi Shayba 13:609

Abu Bakr narrated to us, he said: Hafs narrated to us, from Layth, from Mujahid, who said: The Mothers of the Believers would not observe hijab (seclusion) from the *mukātab* (a slave who has a contract of manumission) as long as there remained of his manumission contract even a *mithqāl* or a dinar.