Musannaf Ibn Abi Shaybah

Hazrat Ata says that zakat is not obligatory on wealth until it reaches twenty dinars; when it reaches twenty dinars, half a dinar is due as zakat on it, and for every additional four dinars, one dirham will be due, until it reaches forty, and for every forty, one dinar is due, and for twenty-four dinars, half a dinar and one dirham are due.