Sunan Ibn Majah, Volume 3, Book 19, Hadith 2520 (DarusSalam) ⓘ
It was narrated from Umm Salamah that the Prophet 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 صلى الله عليه وسلم أَنَّهُ قَالَ " إِذَا كَانَ لإِحْدَاكُنَّ مُكَاتَبٌ وَكَانَ عِنْدَهُ مَا يُؤَدِّي فَلْتَحْتَجِبْ مِنْهُ " .
also referenced under: Hadith 2520 (Reference) & Book 19, Hadith 2616 (In-Book)