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