Umm Salamah (RAA) narrated that The Messenger of Allah ( صلی اللہ علیہ وسلم ) 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.