Musannaf Ibn Abi Shayba 13:2650

Abu Bakr narrated to us, he said: Waki' narrated to us, from Hisham ibn 'Urwah, from his father, from 'Aishah, that Barirah came to her while she was a *mukatabah* (a slave under a contract of manumission). She asked the Prophet, peace and blessings of Allah be upon him, [about it]. He said: "Buy her and emancipate her, for the *wala'* (right of inheritance/patronage) belongs only to the one who emancipates."