ENOfficial
Narrated Umm Salamah, Ummul Mu'minin: The Messenger of Allah (ﷺ) said to us: If one of you has a slave, and he enters into an agreement to purchase his freedom, and can pay the full price, she must veil herself from him.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا مُسَدَّدُ بْنُ مُسَرْهَدٍ، حَدَّثَنَا سُفْيَانُ، عَنِ الزُّهْرِيِّ، عَنْ نَبْهَانَ، مُكَاتَبِ أُمِّ سَلَمَةَ قَالَ سَمِعْتُ أُمَّ سَلَمَةَ، تَقُولُ قَالَ لَنَا رَسُولُ اللَّهِ صلى الله عليه وسلم " إِنْ كَانَ لإِحْدَاكُنَّ مُكَاتَبٌ فَكَانَ عِنْدَهُ مَا يُؤَدِّي فَلْتَحْتَجِبْ مِنْهُ " .
sunnah.com