Sahih Bukhari Volume 003, Book 034, Hadith Number 377.
Narated By 'Urwa : 'Aisha said, "Buraira came to me and said, 'I have agreed with my masters to pay them nine Uqiyas (of gold) (in installments) one Uqiya per year; please help me.' I said, 'I am ready to pay the whole amount now provided your masters agree that your Wala will be for me.' So, Buraira went to her masters and told them about that offer but they refused to accept it. She returned, and at that time, Allah's Apostle was sitting (present). Buraira said, 'I told them of the offer but they did not accept it and insisted on having the Wala.'.' The Prophet heard that." 'Aisha narrated the whole story to the Prophet. He said to her, "Buy her and stipulate that her Wala' would be yours as the Wala' is for the manumitted." 'Aisha did so. Then Allah's Apostle stood up in front of the people, and after glorifying Allah he said, "Amma Badu (i.e. then after)! What about the people who impose conditions which are not in Allah's Book (Laws)? Any condition that is not in Allah's Book (Laws) is invalid even if they were one hundred conditions, for Allah's decisions are the right ones and His conditions are the strong ones (firmer) and the Wala' will be for the manumitted."