Yahya related to me from Ibn Shihab from Salim ibn Abdullah fromhis father that Umar ibn al-Khattab used to take a twentieth from thecereals and olive oil of the Nabatean christians, intending by that toincrease the cargo to Madina. He would take a tenth from pulses.
USC-MSA web (English) reference: Book 17, Hadith 47
Arabic reference: Book 17, Hadith 623