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."
USC-MSA web (English) reference: Book 17, Hadith 9
Arabic reference: Book 17, Hadith 589