Musnad Ahmad 1153:44

'Abd al-A'la narrated to us, from Ma'mar, from al-Zuhri, from 'Urwah, from 'A'ishah, that Barirah came to her seeking her assistance, and she was a *mukātabah* (a slave who had entered into a contract of manumission). 'A'ishah said to her, "Will your masters sell you?" So she went to her masters and mentioned that to them. They said, "No, unless you stipulate for us her *wala'* (the right of inheritance/patronage)." The Prophet, peace and blessings of Allah be upon him, said, "Buy her and emancipate her, for the *wala'* belongs only to the one who emancipates."