Malik related to me from Abdullah ibn Dinar from Abdullah ibn Umar that the Messenger of Allah ﷺ forbade selling or giving away the wala'.

أَنَّ رَسُولَ اللَّهِ ﷺ نَهَى عَنْ بَيْعِ الْوَلاَءِ وَعَنْ هِبَتِهِ