Sayyiduna Mu'adh bin Jabal (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) sent me to the people of Yemen and ordered me: "That I collect a tenth from the lands that the sky waters and from lands with naturally flowing water, and that I collect half a tenth from the lands watered by water-wheels and the like."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7491