Sahih Bukhari Volume 9, Book 83, Hadith Number 47.

Narrated By Abu Huraira : Allah’s Apostle said, “There is no Diya for persons killed by animals or for the one who has been killed accidentally by falling into a well or for the one killed in a mine. And one-fifth of Rikaz (treasures buried before the Islamic era) is to be given to the state.”