Sahih Muslim Book 17, Hadith Number 4242.

Chapter : If the beast does any harm, or if one falls in a mine or a well, there will be no blood-wit for that.

Abu Huraira reported Allah’s Messenger (may peace be upon him) as saying: The wound caused (by falling) in the well, in the mine, and caused by the animal has no requital for it ; and there is one-fifth (for the government) in the buried treasure.