Sayyidah Aishah (may Allah be pleased with her), describing the affair of Khaybar, said: The Prophet (peace and blessings be upon him) used to send Sayyiduna Ibn Rawahah (may Allah be pleased with him) to the Jews, and he would estimate the dates when they ripened, before any of them were eaten. Then the Jews would be given the choice either to take (the fruit) themselves at that estimate or to hand over (the Muslims' half) at that estimate. The Prophet (peace and blessings be upon him) ordered the estimation only so that the zakah could be calculated before the fruit was eaten and distributed.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14016