Musnad Ahmad 30:3120

Waki’ narrated to us, Hammad narrated to us, Muhammad narrated to us, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The animal is 'jibar' (liable for no compensation), the well is 'jibar', and the mine is 'jibar'. And in the 'rikaz' (buried treasure), there is one-fifth."