Sayyiduna Mu'adh bin Jabal (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) said: "On that (land) which the sky waters, or which is rain-fed, or watered by floods, there is a tithe; and on what is watered by drawing water, half a tithe. And this applies to dates, wheat and grains; as for cucumbers, melons, pomegranates and vegetables, the Messenger of Allah (peace and blessings be upon him) has exempted them."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7477