Nafi' narrates from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the zakah of fruits and crops is on what comes from date palms, grapes, wheat, barley and hull-less barley; whatever (produce) is watered by springs or a river, or is rain-fed and watered by the rain, a tenth is due on it, that is, one out of every ten; and whatever is watered by drawing water, half a tenth is due on it, that is, one out of every twenty. And the Prophet (peace and blessings be upon him) sent a letter to the people of Yemen: "Upon al-Harith bin Abd Kulal and the believers of Ma'afir and Hamdan who are with him is zakah on fruits; and on land (produce) watered by springs, a tenth is due; on what the sky waters, a tenth is also due; and on what is watered from wells, half a tenth is due."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7487