From Sahih Bukhari (Hadith 2564):
Narrated `Amra bint `Abdur-Rahman:Barirah went to Aisha, the mother of the faithful believers to seek her help in her emancipation Aisha said to her, "If your masters agree, I will pay them your price in a lump sum and manumit (free) you." Barirah mentioned that offer to her masters but they refused to sell her unless the Wala' was for them. `Aisha told Allah's Messenger (ﷺ) about it. He said, "Buy and manumit her as the Wala' is for the liberator
Reference: Bukhari 2564