Sahih Bukhari Volume 3, Book 46, Hadith Number 736.

Narated By ‘Abdullah bin ‘Umar : ‘Aisha wanted to buy a slave-girl in order to manumit her. The girl’s masters stipulated that her Wala’ would be for them. Allah’s Apostle said (to ‘Aisha), “What they stipulate should not stop you, for the Wala’ is for the liberator.”