Musannaf Ibn Abi Shayba 28:171

Muhammad ibn Bishr narrated to us, he said: Hisham ibn Sa’d narrated to us, he said: ‘Amr ibn Shu’ayb narrated to me, from his father, from his grandfather, from the Prophet, peace and blessings of Allah be upon him, he said: "In buried treasure (rikaz), there is one-fifth."