Sahih Bukhari Volume 003, Book 050, Hadith Number 878.
Narated By Urwa : 'Aisha told me that Buraira came to seek her help in writing for emancipation and at that time she had not paid any part of her price. 'Aisha said to her, "Go to your masters and if they agree that I will pay your price (and free you) on condition that your Wala' will be for me, I will pay the money." Buraira told her masters about that, but they refused, and said, "If 'Aisha wants to do a favor she could, but your Wala will be for us." 'Aisha informed Allah's Apostle of that and he said to her, "Buy and manumit Buraira as the Wala' will go to the manumitted."