Abd al-Rahman bin Mas'ud bin Niyar narrates that Sayyiduna Sahl bin Abi Hathmah (may Allah be pleased with him) was in a gathering of ours, and he told us that the Prophet (peace and blessings be upon him) used to say to those who estimated the fruit (for zakah): "Take (according to your estimate) and leave a third; and if you cannot leave a third - or he said: do not find - then leave a quarter."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5605