Musnad Ahmad 30:2730

Hajjaj narrated to us, Shu'bah narrated to us, from Muhammad ibn Ziyad, from Abu Hurayrah, who said: I heard the Messenger of Allah (ﷺ) — or he said: Abu al-Qasim (ﷺ) said —: "The injury caused by an animal is exempt (from liability), the well is exempt, and the mine is exempt, and in the treasure (rikaz) is one-fifth." Shu'bah said: "I have not heard anyone say 'rikaz' (plural: raka'iz) except him."