Musannaf Ibn Abi Shayba 35:159

Ibn ‘Ulayyah narrated to us, from ‘Abd al-Rahman ibn Ishaq, from al-Zuhri, from Sa‘id ibn al-Musayyib: That the Prophet, peace and blessings of Allah be upon him, commanded ‘Attab ibn Asid to estimate the grapes just as the date palms are estimated, so that its Zakat is paid as raisins, just as the Zakat of the date palms is paid as dates. That is the Sunnah of the Prophet, peace and blessings of Allah be upon him, regarding date palms and grapes.