Sahih Muslim Book 5, Hadith Number 2158.

Abu Sa’id al-Khudri reported that when Mu’awiya prescribed half a sa’ of wheat equal to one sa’ of dates, he (Abu Sa’id al-Khudri) objected to it, and said: I would take out (Sadaqat-ul-Fitr) but that which I used to bring forth during the lifetime of the Messenger of Allah (may peace be upon him) one sa’ of dates, or one sa’ of raisins, or one sa’ of barley, or one sa’ of cheese.