Lady Aisha (may Allah be pleased with her) was once mentioning Khaybar, she said: The Prophet Muhammad (peace be upon him) sent Abdullah bin Rawahah (may Allah be pleased with him) to the Jews. When the fruits ripened, he estimated the yield of the date trees. Then he gave the Jews the choice to either take the produce according to that estimate or pay according to that estimate. The Prophet (peace be upon him) had ordered the estimation so that the calculation of zakat could be done before the fruits ripened and it could be set aside. The same narration is also transmitted from Abu Hurairah (may Allah be pleased with him) with another chain of narration. Imam Malik has transmitted it as a mursal narration.