(4) باب بَيْعِ الْمُكَاتَبِ إِذَا رَضِيَ
Narrated `Amra bint `Abdur-Rahman:
Buraira 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 you." Buraira 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: Sahih al-Bukhari 2564
In-book reference: Book 50, Hadith 5
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 3, Book 46, Hadith 738