Ibn Umar (may Allah be pleased with him) narrates from Aishah (may Allah be pleased with her) that Aishah (may Allah be pleased with her) wanted to buy a slave girl in order to free her. Her owners said: We will sell her, but her wala' will be ours. Aishah (may Allah be pleased with her) mentioned this to the Prophet (peace and blessings be upon him), and he (peace and blessings be upon him) said: "Do not let this stop you from buying her. Wala' belongs to the one who frees."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الولاء / 21448