Musnad Ahmad 30:2204

Muhammad ibn Ja'far narrated to us, he said: Hisham narrated to us, from Muhammad, from Abu Hurayrah, from the Prophet, may Allah bless him and grant him peace, that he said: "The injury caused by an animal is exempt [from liability]. The well is exempt [from liability]. The mine is exempt [from liability]. And in the buried treasure, there is one-fifth."