Sahih Bukhari
Obligatory Charity Tax (Zakat)
Volume: 002 / Book Number: 024 / Hadith Number: 560
Narrated Salim bin 'Abdullah from his father:
The Prophet said, "On a land irrigated by rain water or by natural water channels or if the land is wet due to a near by water channel Ushr (i.e. one-tenth) is compulsory (as Zakat); and on the land irrigated by the well, half of an Ushr (i.e. one-twentieth) is compulsory (as Zakat on the yield of the land)."