Musnad Abu Ya'la al-Mawsili 160:214

Narrated to us Dawud ibn 'Amr, narrated to us Habban ibn 'Ali, from 'Abdullah ibn Sa'id, from his father, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The Rikaz is the gold which grows from the earth."