Musnad Ahmad 1153:1287

'Abd al-Razzaq narrated to us, Ibn Jurayj informed us, he said: I was informed from Ibn Shihab, from 'Urwah, from 'Aishah, that she said while mentioning the affair of Khaybar: "The Prophet, may God bless him and grant him peace, used to send Ibn Rawahah to the Jews, and he would estimate for them the palm trees when they ripened, before any of it was eaten. Then the Jews were given the choice to take it at that estimate, or to hand it over to them at that [estimate]. The command of the Prophet, may God bless him and grant him peace, for the estimation was only so that he could calculate the Zakat before the fruit was eaten and distributed."