Sayyiduna Sahl bin Abi Hathmah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: When you estimate the crop, then after it is harvested (collect this amount of zakat, but) leave one third. If you do not leave that much, then leave one fourth.
Hadith Referenceالفتح الربانی / كتاب الزكاة / 3421
Benefits: … The ahadith of this chapter indicate the legitimacy of estimating the yield of dates, grapes, and similar crops. According to Shafi'i and Ahmad, it is recommended (mustahabb) to estimate the yield of these crops. According to Malik, Shurayh, Abu Ja'far, and some of the Zahiri scholars, it is obligatory (wajib). Nawawi has said that for crops such as dates and grapes, in which the payment of 'ushr (one-tenth) is obligatory, it is recommended (mustahabb) to estimate the yield. In this chapter, there is a statement of a great facilitative concession for landowners, and that is: government representatives, after the crop has ripened, should estimate the yield and then, in light of the one-tenth or one-twentieth share, determine the total amount of zakat due from the landowner. In this way, landowners are saved from the detailed and overall accounting of the entire crop, because there are many crops of which some portion is fed to animals, some portion is kept by the owner for his household, some portion is given to cobblers and barbers, and some portion is sold to traders.