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.

وَعَنْ أَُمِّ سَلَمَةَ ‏-رَضِيَ اَللَّهُ عَنْهَا‏- قَالَتْ: قَالَ رَسُولُ اَللَّهِ ‏- صلى الله عليه وسلم ‏-{ إِذَا كَانَ لِإِحْدَاكُنَّ مُكَاتَبٌ, وَكَانَ عِنْدَهُ مَا يُؤَدِّي, فَلْتَحْتَجِبْ مِنْهُ } رَوَاهُ اَلْخَمْسَة ُ [1]‏ وَصَحَّحَهُ اَلتِّرْمِذِيّ ُ [2]‏ .‏