Musnad Abu Ya'la al-Mawsili 149:79

Suwayd ibn Sa'id narrated to us, from Malik, from Hisham ibn 'Urwah, from his father, from 'Aishah, that she said: Barirah came to me and said: "I have entered into a contract of manumission (kitabah) with my masters for nine awqiyahs, one awqiyah every year, so assist me." 'Aishah said to her: "If your masters like for me to pay it to them in one lump sum, and your wala' (right of inheritance/patronage) be for me, I will do it." So Barirah went to her masters and told them, but they refused, insisting that the wala' be for them. The Messenger of Allah (ﷺ) heard of that and asked her about it, and 'Aishah informed him. The Messenger of Allah (ﷺ) said: "Take her and stipulate the wala' for them, for the wala' belongs only to the one who emancipates." 'Aishah said: The Messenger of Allah (ﷺ) stood among the people, praised Allah, and then said: "To proceed: What is the matter with people who stipulate conditions that are not in the Book of Allah? Whatever is not in the Book of Allah is invalid, even if it were a hundred conditions." He said: "The decree of Allah is more deserving, and the condition of Allah is more binding, and the wala' belongs only to the one who emancipates."