Sahih Muslim Book 17, Hadith Number 4239.

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: No requital is payable for a wound caused by an animal, for (falling into) a well and a mine, and one-fifth (is the share of the government) in the buried treasure (treasure-trove).