Musa bin Talhah (may Allah have mercy on him) narrates from Sayyiduna Mu'adh bin Jabal (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) made a tenth ('ushr, as zakah) obligatory on grains watered by rain, on ba'l (land watered through its roots by the moisture of the soil), and on those watered by springs. As for melons, cucumbers, herbs, green vegetables, pomegranates and sugarcane, the Messenger of Allah (peace and blessings be upon him) exempted them (from zakah).
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17450