Sayyidah Umm Salamah (may Allah be pleased with her) says that the Messenger of Allah (peace and blessings be upon him) said: "When the mukatab of any one of you has the price with him, then she should observe hijab from him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21661