Sayyiduna Abu Hurayrah (may Allah be pleased with him) says that Sayyidah Aishah (may Allah be pleased with her) intended to buy a slave girl and free her, but her people refused unless the wala' would be theirs. So Sayyidah Aishah (may Allah be pleased with her) mentioned this to the Messenger of Allah (peace and blessings be upon him), and he (peace and blessings be upon him) said: "Let nothing stop you from buying this slave girl, for the wala' belongs to the one who frees."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21732