Sahih Bukhari Voluume 003, Book 050, Hadith Number 889.
Narated By Urwa : 'Aisha said, "Buraira came to me and said, 'My people (masters) have written the contract for my emancipation for nine Awaq) of gold) to be paid in yearly installments, one Uqiyya per year; so help me." 'Aisha said (to her), "If your masters agree, I will pay them the whole sum provided the Wala will be for me." Buraira went to her masters and told them about it, but they refused the offer and she returned from them while Allah's Apostles was sitting. She said, "I presented the offer to them, but they refused unless the Wala' would be for them." When the Prophet heard that and 'Aisha told him about It, he said to her, "Buy Buraira and let them stipulate that her Wala' will be for them, as the Wala' is for the manumitted." 'Aisha did so. After that Allah's Apostle got up amidst the people, Glorified and Praised Allah and said, "What is wrong with some people who stipulate things which are not in Allah's Laws? Any condition which is not in Allah's Laws is invalid even if there were a hundred such conditions. Allah's Rules are the most valid and Allah's Conditions are the most solid. The Wala is for the manumitted."