Musnad Abu Ya'la al-Mawsili 209:78

Abu Khaythama narrated to us, Sufyan ibn 'Uyaynah narrated to us, from al-Zuhri, from Nabhan, from Umm Salamah, who mentioned the Prophet (may Allah bless him and grant him peace), saying: "If one of you has a *mukātab* (a slave with a contract of manumission) and he possesses what he needs to fulfill [the payment], then she must observe hijab (seclusion) from him."