Musnad Ahmad 1153:2316

Ya'qub narrated to us, he said: Ibn Akhi Ibn Shihab narrated to us, from his uncle, who said: 'Urwah ibn al-Zubayr informed me that 'Aishah, the wife of the Prophet, may Allah bless him and grant him peace, informed him that Barirah entered upon her seeking her help regarding her *kitabah* (contract of manumission). 'Aishah said to her, and she desired her [freedom]: "Do you see, if I were to pay your masters the amount that is upon you in one payment, would they do that and free you, so that you would be my *mawlah* (freed slave)?" Barirah went to her masters and presented that to them, but they said: "No, unless your *wala'* (right of inheritance/patronage) is for us." 'Aishah said: Then the Messenger of Allah, may Allah bless him and grant him peace, entered upon me, and I mentioned that to him. The Messenger of Allah, may Allah bless him and grant him peace, said: "Buy [her] and free [her], for the *wala'* belongs to the one who frees." Then the Messenger of Allah, may Allah bless him and grant him peace, stood up in the evening and said: "What is the matter with people who stipulate conditions that are not in the Book of Allah? Whoever stipulates a condition that is not in the Book of Allah, it is not valid for him, even if he stipulates it a hundred times. The condition of Allah is more rightful and more binding."