Abu Bakr bin Muhammad bin Amr bin Hazm narrates from his father, and he from his grandfather, that he (peace and blessings be upon him) wrote in the letter he sent to the people of Yemen: "Land that is watered by the sky, or is rain-fed, or is elevated land, a tenth is due on it when its produce reaches five wasq; and land that is watered by hand or by water drawn up and poured, half a tenth is due on it when its produce reaches five wasq."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7425