From Sahih Muslim (Hadith 2285):
Sa'd b. Abu Sarh heard Abu Sa'id al-Khudri as saying:We, on behalf of young or old, free or slave, used to take out the Zakat of Fitr while the Messenger of Allah (may peace he upon him) was among us, in three kinds, one sa' of dates, one sa' of cheese, or one sa' of barley, and we continued to take that out till the time of Mu'awiya, for he saw that two mudds of wheat were equal to one sa' of dates. Abu Sa'id sald: I would continue to take that out as before (i e. one sa' of wheat)
Reference: Muslim 2285