Musnad Ahmad 27:1047

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, 'Abdullah ibn Dinar narrated to us, he said: I heard Ibn 'Umar saying: "The Messenger of Allah (peace and blessings of Allah be upon him) forbade the selling of *al-wala'* (the right of inheritance/patronage) and the gifting of it."