Musnad Ahmad 27:1399

'Affan narrated to us, Shu'bah narrated to us, 'Abdullah ibn Dinar informed me, from Ibn 'Umar, from the Prophet (peace and blessings of Allah be upon him), who said: "He forbade the selling of *wala'* (patronage rights) and the gifting of it." He said: I said: "Did you hear it from Ibn 'Umar?" He said: "Yes, and his son Hamzah asked him about it."