Sahih Bukhari Volume 3, Book 46, Hadith Number 713.

Narated By ‘Aisha : I bought Buraira but her masters put the condition that her Wala’ would be for them. I told the Prophet about it. He said (to me), “Manumit her as her Wala’ will be for the one who pays the price.” So, I manumitted her. The Prophet called Buraira and gave her the option of either staying with her husband or leaving him. She said, “Even if he gave me so much money, I would not stay with him,” and so she preferred her freedom to her husband.