Sayyidah Aishah (may Allah be pleased with her) says that Sayyidah Barirah (may Allah be pleased with her) said: My people have made a contract of manumission with me for nine uqiyahs over nine years, one uqiyah each year, so help me. Sayyidah Aishah (may Allah be pleased with her) says: If your owners wish, I will pay the amount in one go and free you, and the wala' will be mine. She mentioned this to her people, but they refused to give up the wala'. She came to me and mentioned it, so I scolded her, and she said: By Allah, not in that case. She says: the Messenger of Allah (peace and blessings be upon him) heard, and I informed him. He (peace and blessings be upon him) said: "Buy her and free her, and stipulate the wala' for them, for the wala' belongs to the one who frees." She says: I did so. Then in the evening the Messenger of Allah (peace and blessings be upon him) delivered a sermon; he praised and extolled Allah as He deserves, then said: "What is the matter with people that they stipulate conditions that are not in the Book of Allah? Any condition that is not in the Book of Allah is void, even if there are a hundred conditions. The Book of Allah is more deserving, and the conditions of Allah are firmer. What is the matter with men who say: Free him, and the wala' is mine, when the wala' belongs only to the one who frees."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10845