Sahih al-Bukhari 6756
Sahih al-Bukhari · Book 85: Laws of Inheritance (Al-Faraa'id) · The sin of the freed slave who denies his master who has freed him.
Narrated Ibn `Umar:
حَدَّثَنَا أَبُو نُعَيْمٍ ، حَدَّثَنَا سُفْيَانُ ، عَنْ عَبْدِ اللَّهِ بْنِ دِينَارٍ ، عَنِ ابْنِ عُمَرَ ـ رضى الله عنهما ـ قَالَ نَهَى النَّبِيُّ صلى الله عليه وسلم عَنْ بَيْعِ الْوَلاَءِ وَعَنْ هِبَتِهِ.
The Prophet (ﷺ) forbade the selling of the Wala' (of slaves) or giving it as a present.