Musnad Ahmad 1156:184

Muhammad ibn Ja'far narrated to us, he said: Ma'mar narrated to us, Ibn Shihab narrated to us, from Nabhan, from Umm Salamah, she said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "When the *mukātab* (a slave who has a contract of manumission) finds what he can pay off [his contract with], then veil yourselves from him."