Musnad Ahmad 1156:157

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from al-Zuhri, Nabhan—the *mukātab* (slave under a contract of manumission) of Umm Salamah—narrated to me, he said: I was leading her [Umm Salamah] in al-Bayda', or he said: in al-Abwa'. She said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: "When the *mukātab* possesses what he can pay off [his contract with], then veil yourself from him."