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." (FR)
English reference (FR): Vol. 3, Book 19, Hadith 2520 (FR)
Arabic reference (FR): Book 19, Hadith 2616 (FR)