Musnad Ahmad 27:1311

'Affan narrated to us, Hammam narrated to us, I heard Nafi' claiming that Ibn 'Umar narrated to him that 'Aisha bargained for Barirah. The Prophet, peace and blessings of Allah be upon him, went out to the prayer. When he returned, she said: "They refused to sell her to me unless they stipulate the right of inheritance (al-wala')." The Prophet, peace and blessings of Allah be upon him, said: "The right of inheritance belongs only to the one who emancipated."