Abd al-Rahman bin Mas'ud says: Sahl bin Abi Hathmah (may Allah be pleased with him) came to our gathering and said: the Messenger of Allah (peace and blessings be upon him) commanded me: "When you estimate, leave a third; and if you do not leave a third, then leave a quarter."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7443