The Prophet (peace and blessings be upon him) said: "The zakah of grapes is to be estimated just as dates are estimated, then its zakah is to be paid in raisins, just as the zakah of dates is paid in tamr, that is, dried dates."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قسم الصدقات / 13125