Muwatta Imam Malik Book of Setting Free and Wala', Hadith 20
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'.
وَحَدَّثَنِي مَالِكٌ، عَنْ عَبْدِ اللَّهِ بْنِ دِينَارٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ ﷺ نَهَى عَنْ بَيْعِ الْوَلاَءِ وَعَنْ هِبَتِهِ .