Sa'id bin al-Musayyib (may Allah have mercy on him) said that the Messenger of Allah (peace and blessings be upon him) ordered Attab bin Asid (may Allah be pleased with him) to estimate grapes just as dates are estimated. Then he said: "Pay its zakah in raisins, just as the zakah of dates is paid in dried dates." He said: This is the Sunnah of the Messenger of Allah (peace and blessings be upon him) concerning dates and grapes.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7434