Sayyiduna Abu Umamah bin Sahl (may Allah be pleased with him) stated in the gathering of Sa'id bin al-Musayyib (may Allah have mercy on him) that the Sunnah has been established that zakah is not to be collected from dates and grapes until their estimate reaches five wasq. Al-Zuhri (may Allah have mercy on him) said: I have not known any produce to be estimated except dates and grapes.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7435