It was narrated from Umm Salamah that the Prophet ﷺ said:
“If anyone of you (women) has a Mukatab, and he has enough (wealth) to pay off (his contact of manumission), she must veil herself from him.”

حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا سُفْيَانُ بْنُ عُيَيْنَةَ، عَنِ الزُّهْرِيِّ، عَنْ نَبْهَانَ، - مَوْلَى أُمِّ سَلَمَةَ - عَنْ أُمِّ سَلَمَةَ، أَنَّهَا أَخْبَرَتْ عَنِ النَّبِيِّ ﷺ أَنَّهُ قَالَ " إِذَا كَانَ لإِحْدَاكُنَّ مُكَاتَبٌ وَكَانَ عِنْدَهُ مَا يُؤَدِّي فَلْتَحْتَجِبْ مِنْهُ " .