Bushayr bin Yasar (may Allah have mercy on him) says that Umar bin al-Khattab (may Allah be pleased with him) used to send Abu Hathmah (may Allah be pleased with him) as an estimator to estimate the date palms, and he would instruct him: When you find the people in their orchard, leave for them what they eat and do not estimate it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7445