Maliks Muwatta Book 17, Hadith Number 9.
Section : The Zakat on Buried Treasure (Rikaz).
Yahya related to me from Malik from Ibn Shihab from Said ibn al-Musayyab and from Abu Salama ibn Abd ar-Rahman from Abu Hurayra that the Messenger of Allah, may Allah bless him and grant him peace, said, “There is a tax of a fifth on buried treasure.”
Malik said, “The position which we are agreed upon, and which I have heard the people of knowledge mentioning, is that rikaz refers to treasure which has been found which was buried during the jahiliyya, as long as neither capital is required, nor expense, great labour or inconvenience incurred in recovering it. If capital is required or great labour is incurred, or on one occasion the mark is hit and on another it is missed, then it is not rikaz.”