Sunan Abi Dawud, Book 29, Hadith 3917 (USC-MSA) ⓘ
Narrated Umm Salamah, Ummul Mu'minin: The Messenger of Allah 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.
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 " .
also referenced under: Hadith 3928 (DarusSalam) & Hadith 3917 (Hasan)