The Messenger of Allah ﷺ said, "On buried treasure, a fifth is due."
أن رسول الله ﷺ قال: في الركاز الخمس.
No references are available.
Imam Malik related it in the Book of Zakat (9), from Ibn Shihab, from Saeed bin Musayab, and from Abu Salama bin Abdul-Rahman, from Abu Hurairah, and he mentioned it. Bukhari related it in the Book of Zakat (1499), from Abdullah bin Yusuf, who said, Malik informed us. Muslim related it in the Book of Hudud (1710) through other routes from Abu Hurairah. See the hadith with a fuller wording in the Book of Diyat.