Umm Salamah (RAA) narrated that The Messenger of Allah (ﷺ) said:
"When a slave of one of your women has made an agreement to pay for his freedom (i.e. he is a Mukatib) and can pay the full price, she must veil herself from him." Related by Ahmad and the four Imams. At-Tirmidhi graded it as Sahih. (IT)
English reference (IT): Book 15, Hadith 1476 (IT)
Arabic reference (IT): Book 15, Hadith 1432 (IT)