Sahih Bukhari | Volume 3 | Hadith Number 378
Narrated by 'Abdullah bin 'Umar
'Aisha, (mother of the faithful believers) wanted to buy a slave girl and manumit her, but her masters said that they would sell her only on the condition that her Wala' would be for them. 'Aisha told Allah's Apostle of that. He said, "What they stipulate should not hinder you from buying her, as the Wala' is for the manumitted."