Maliks Muwatta Book 17, Hadith Number 34.
Section : The Zakat on Estimated Yields of Date Palms and Vines.
Yahya related to me from Malik from a reliable source from Sulayman ibn Yasar and from Busr ibn Said that the Messenger of Allah, may Allah bless him and grant him peace, said, “On land that is watered by rain or springs or any natural means there is (zakat to pay of) a tenth. On irrigated land there is (zakat of) a twentieth (to pay).”