Volume 8 · Book 79 · Hadith 708
Narrated 'Aisha: that she intended to buy Barira (a slave girl) and her masters stipulated that they would have her Wala'. When 'Aisha mentioned that to the Prophet ; he said, "Buy her, for the Wala' is for the one who manumits."
— Sahih Bukhari, Volume 8, Book 79: Expiation for Unfulfilled Oaths, Hadith #708