Musannaf Ibn Abi Shayba 13:623

Abu Bakr narrated to us, he said: Ibn ‘Uyaynah narrated to us, from al-Zuhri, from Nabhān, from Umm Salamah, she said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If one of you has a *mukātab* (a slave with a contract of manumission) and he possesses what he needs to pay off [the contract], then she must observe *hijāb* (seclusion) from him."