Maliks Muwatta Book 38, Hadith Number 19.
Section : The Right of the One who Sets Free to the Wala’.
Malik related to me from Yahya ibn Said from Amra bint Abd ar-Rahman that Barira came asking the help of A’isha, umm al-muminin. A’isha said, “If your masters agree that I pay them your price in one lump sum and set you free I will do it.” Barira mentioned that to her masters and they said, “No, not unless your wala’ is ours.” Yahya ibn Said added that Amra bint Abd ar-Rahman claimed that A’isha mentioned that to the Messenger of Allah, may Allah bless him and grant him peace, and the Messenger of Allah, may Allah bless him and grant him peace said, “Buy her and set her free. The wala’ only belongs to the one who sets free.”