Sahl bin Abi Hathmah (RAA) narrated, "We were commanded by the Messenger of Allah (ﷺ), 'Whenever you assess the amount of something, estimate the [Zakah] and ignore one-third. If you do not, then leave (at least) one fourth." Related by the five Imams, except for Ibn Majah. (IT)
English reference (IT): Book 4, Hadith 640 (IT)
Arabic reference (IT): Book 4, Hadith 618 (IT)