And the Messenger of Allah, peace and blessings be upon him, said: The wound caused by a beast is without liability, the blood of one who dies by falling into a well is without liability, the blood of one who dies in a mine is also without liability, and for buried treasure that someone finds, one fifth (khums) is obligatory.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 8252