Sahih Bukhari Voluume 003, Book 050, Hadith Number 893.
Narated By Amra : 'Aisha said that Buraira came to seek her help in the writing of her emancipation. 'Aisha said to her, "If you wish, I will pay your masters (your price) and the wala' will be for me." When Allah's Apostle came, she told him about it. The Prophet said to her, "Buy her (i.e. Buraira) and manumit her, for the Wala is for the one who manumits." Then Allah's Apostle ascended the pulpit and said, "What about those people who stipulate conditions which are not in Allah's Laws? Whoever stipulates such conditions as are not in Allah's Laws, then those conditions are invalid even if he stipulated a hundred such conditions."