Musnad Ahmad 1156:3

Sufyan ibn 'Uyaynah narrated to us, from al-Zuhri, from Nabhan, from Umm Salamah, who mentioned that the Prophet, peace and blessings of Allah be upon him, said: "If one of you has a mukatab (a slave with a contract of manumission), and he possesses what is sufficient to pay off [his contract], then let her observe hijab (seclusion) from him."