Yahya related to me from Malik from a reliable source fromSulayman ibn Yasar and from Busr ibn Said that the Messenger of Allah,may Allah bless him and grant him peace, said, "On land that iswatered by rain or springs or any natural means there is (zakat to payof) a tenth. On irrigated land there is (zakat of) a twentieth (topay)."
USC-MSA web (English) reference: Book 17, Hadith 34
Arabic reference: Book 17, Hadith 611