It is narrated from Sayyiduna Sa'id bin al-Musayyib (may Allah have mercy on him) that the Messenger of Allah (ﷺ) made two mudd of wheat obligatory as Fitrana. It is narrated from Imam Muzani (may Allah have mercy on him) that Imam Shafi'i (may Allah have mercy on him) said: "The hadith of two mudd is based on error." (That is, the correct amount is a full sa', which contains four mudd.)
Hadith Referenceالسنن المأثورة / باب ما جاء في صدقة الفطر / 368