Salim bin Abdullah narrates from his father (Sayyiduna Abdullah bin Umar, may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Whatever is watered by the sky or by springs, or is rain-fed, a tenth is due on it; and whatever is watered by drawing water, half a tenth is due on it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7485