Musnad Imam Shafi'i

It is narrated from Nafi' (may Allah be pleased with him) that Abdullah bin Umar (may Allah be pleased with him) used to say: The charity (zakat) of fruits and crops—whether dates, grapes, crops, barley, or sult (a type of barley grown in Hijaz that resembles wheat)—which derive moisture from the earth, or are irrigated by a canal, or watered by a spring, or irrigated by rain, in these there is a tenth (one-tenth) due. For every ten, one part will be taken. And for that which is irrigated by drawing water with buckets or by bringing water with animals, there is half of a tenth (one-twentieth) due; for every twenty, one part will be taken. For the field irrigated by drawing water with buckets or by bringing water with animals, one-twentieth (of the produce) will be taken.