Musnad Ahmad 27:407

Yazid narrated to us, from Hammam, from Nafi’, from Ibn Umar, that Aisha wanted to purchase Barirah, but her people refused to sell her unless the right of inheritance (wala’) belonged to them. Aisha mentioned that to the Prophet, may Allah bless him and grant him peace. The Messenger of Allah, may Allah bless him and grant him peace, said: "Purchase her and emancipate her, for the right of inheritance (wala’) belongs only to the one who paid the price."