Sayyiduna Attab bin Asid (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Grapes are to be estimated just as dates are estimated, and their zakah is to be taken in raisins, just as the zakah of dates is taken."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7432