Al-Muwatta Hadith 38.17

Malik related to me from Hisham ibn Urwa from his father that A'isha, the wife of the Prophet, may Allah bless him and grant him peace, said, "Barira came to me and said, 'I have written myself as mukatab for my people for nine uqiyas, one uqiya per year, so help me.' A'isha said, 'If your people agree that I pay it all to them for you, and that if I pay it, your wala' is mine, then I will do it.' Barira went to her masters and told them that and they didn't agree. She came back from her masters while the Messenger of Allah, may Allah bless him and grant him peace, was sitting. She said to A'isha, 'I offered that to them and they refused me unless they had the wala'.' The Messenger of Allah, may Allah bless him and grant him peace, heard that and asked her about it A'isha told him and the Messenger of Allah, may Allah bless him and grant him peace, said, 'Take her and stipulate that the wala' is yours, for the wala' is for the one who sets free.' So A'isha did that and then the Messenger of Allah, may Allah bless him and grant him peace, stood up in front of the people, and praised Allah and gave thanks to Him. Then he said, 'What is wrong with the people who make conditions which are not in the Book of Allah? Any condition which is not in the Book of Allah is invalid even if it is a hundred conditions. The decree of Allah is truer and the conditions of Allah are firmer, and the wala' only belongs to the one who sets free.' "