(2) باب مَا يَجُوزُ مِنْ شُرُوطِ الْمُكَاتَبِ، وَمَنِ اشْتَرَطَ شَرْطًا لَيْسَ فِي كِتَابِ اللَّهِ
Narrated `Abdullah bin `Umar:
Aisha wanted to buy a slave-girl in order to manumit her. The girl's masters stipulated that her Wala' would be for them. Allah's Messenger (ﷺ) said (to `Aisha), "What they stipulate should not stop you, for the Wala' is for the liberator."
Reference: Sahih al-Bukhari 2562
In-book reference: Book 50, Hadith 3
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 3, Book 46, Hadith 736