Maliks Muwatta Book 17, Hadith Number 47.
Section : Tithes for the People of Dhimma.
Yahya related to me from Ibn Shihab from Salim ibn Abdullah from his father that Umar ibn al-Khattab used to take a twentieth from the cereals and olive oil of the Nabatean christians, intending by that to increase the cargo to Madina. He would take a tenth from pulses.