Ibn Abi al-Shuwarab محمد بن عبد الملك بن أبي الشوارب · The Book on Zakat

the Messenger of Allah said: "I have exempted charity on horses and slaves. So bring charity for silver, one Dirham for every forty Dirham. There is nothing for me (to collect) on one hundred and ninety Dirham, so when it reaches two hundred, then five Dirham of it (are due)."