Al-Hasan al-Basri narrates: This narration has reached me that the Prophet (peace and blessings be upon him) said: "Dying by falling into a mine is not compensated; dying by falling from an animal is not compensated; dying by falling into a well is not compensated; and on buried treasure one fifth is due."
Ahmad recorded it as a mursal narration, and its chain is sahih (authentic).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الزكاة / 4418