Musannaf Ibn Abi Shayba 28:170

Ibn Idris narrated to us, from Layth, from Abu Qays Abd al-Rahman ibn Tharwan, from Huzayl, who said: A man came to Abdullah and said, "I found two hundred dirhams." Abdullah said, "I see that the wealth of the Muslims is being lost. I consider this to be the Zakat of a warrior's wealth. So, pay one-fifth of it into the public treasury (Bayt al-Mal), and you may have what remains."