It is narrated from Zuraiq bin Hakeem that Umar bin Abdul Aziz, may Allah have mercy on him, wrote to him (a letter) stating that whatever merchandise of the Muslims passes by you, take zakat from their trade goods. For every forty dinars, take one dinar. If it is less than that, then take zakat in proportion, up to twenty dinars. If it is less by a third of a dinar, then leave it and do not take zakat from that amount.
Hadith Referenceمسند الإمام الشافعي / كتاب الزكاة / 712
Hadith Takhrijاسناده صحیح اخرجه البيهقي في المعرفة السنن والآثار (2343) ، (2368) - وعبدالرزاق (10116)، (19278) وابن أبي شيبة (9878)۔