Abd al-Rahman bin al-Qasim narrates from his father that Sayyidah Aishah (may Allah be pleased with her) bought Barirah from some people of the Ansar, and they made the condition of wala'. The Messenger of Allah (peace and blessings be upon him) said: "Wala' belongs to the one who has bestowed the favour (of freedom)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الولاء / 21451