Sahih Bukhari Volume 3, Book 46, Hadith Number 737.
Narated By ‘Aisha : Buraira came (to ‘Aisha) and said, “I have made a contract of emancipation with my masters for nine Uqiyas (of gold) to be paid in yearly installments. Therefore, I seek your help.” ‘Aisha said, “If your masters agree, I will pay them the sum at once and free you on condition that your Wala’ will be for me.” Buraira went to her masters but they refused that offer. She (came back) and said, “I presented to them the offer but they refused, unless the Wala’ was for them.” Allah’s Apostle heard of that and asked me about it, and I told him about it. On that he said, “Buy and manumit her and stipulate that the Wala’ should be for you, as Wala’ is for the liberator.” ‘Aisha added, “Allah’s Apostle then got up amongst the people, Glorified and Praised Allah, and said, ‘Then after: What about some people who impose conditions which are not present in Allah’s Laws? So, any condition which is not present in Allah’s Laws is invalid even if they were one-hundred conditions. Allah’s ordinance is the truth, and Allah’s condition is stronger and more solid. Why do some men from you say, O so-and-so! manumit the slave but the Wala will be for me? Verily, the Wala is for the liberator.”