It is narrated from Sayyiduna Attab bin Asid (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said concerning the zakah of grapevines: "Their yield is to be estimated just as the yield of date palms is estimated; then their zakah is to be paid in raisins, just as the zakah of date palms is paid in dates."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14496