Sayyiduna Sahl bin Abi Hathmah (may Allah be pleased with him) says that Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) sent him to estimate the dates and said: When you come to a land, estimate it and leave for them as much as they eat.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7446