From Sahih Bukhari (Hadith 2156):
Narrated `Abdullah bin `Umar:Aisha wanted to buy Barirah and he (the Prophet) went out for the prayer. When he returned, she told him that they (her masters) refused to sell her except on the condition that her Wala' would go to them. The Prophet (ﷺ) replied, 'The Wala' would go to him who manumits.' " Hammam asked Nafi` whether her (Barirah's) husband was a free man or a slave. He replied that he did not know
Reference: Bukhari 2156