Sayyiduna Abu Umamah bin Sahl bin Hunayf (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 date palms until their estimated yield reaches five wasq."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7426