It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "(The damage done by) a dumb animal is not compensable, (the damage arising from working in) a mine is not compensable, (the damage done by) a grazing animal (is also) not compensable, and in buried treasure (rikaz) there is one fifth (khums)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10003