Musannaf Ibn Abi Shaybah

Hazrat Razeeq narrates that Hazrat Umar bin Abdul Aziz wrote to me saying: The dhimmi traders who pass by you, and whose goods are declared and circulate in trade, you should collect one dinar on every twenty dinars, and if it is less than that, then collect in proportion to the decrease, until it reaches ten. Then, if it is even three dinars less than that, then leave it—do not collect anything, and write for them a clearance from those who (in the future) are collectors.