It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) decreed that "one fifth of rikaz (buried treasure) is obligatory to be deposited in the Bayt al-Mal (public treasury)."
Hadith Referenceمسند احمد / ومن مسند بني هاشم / 3276M
Hadith Gradingحکم دارالسلام:صحيح لغيره، رواية سماك عن عكرمة مضطربة