Sahih Muslim Book 5, Hadith Number 2155.

Abd Sa’id al-Khudri reported: We, on behalf of every young or old, free man or slave (amongst us), used to take out during the lifetime of the Messenger of Allah (may peace be upon him) as the Zakat of Fitr one sa’ of grain, or one sa’ of cheese or one sa’ of raisins. And we continued taking out these till Mu’awiya b. Abu Sufyan came to us for pilgrimage or ‘Umra, and addressed the people on the pulpit and said to them: I see that two mudds of zakat out of the wheat (red) of Syria is equal to one sa’ of dates. So the people accepted it. But Abu Sa’id said: I would continue to take out as I used to take out (before, i.e. one sa’) as long as I live.