The Prophet (ﷺ) said: A fifth is payable on buried treasure.
Sunan Abi Dawud Book 20, Hadith 158
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Sunan Abi Dawud Book 20, Hadith 157 Next Hadith → Al hasan said “Rikaz means treasure buried in pre Islamic times.”

Sunan Abi Dawud Book 20, Hadith 159