Sa'id bin al-Musayyib (may Allah have mercy on him) says that the Messenger of Allah (peace and blessings be upon him) said: "Sadaqat al-fitr is two mudds of wheat." Imam al-Shafi'i (may Allah have mercy on him) says: The hadith of the two mudds is an "error." The Shaykh says: It is as he said; the established reports indicate that estimating it at two mudds happened after the Messenger of Allah (peace and blessings be upon him). We have narrated the permissibility of giving half a sa' of wheat in sadaqat al-fitr from Abu Bakr al-Siddiq and Uthman bin Affan (may Allah be pleased with them both), from Jabir bin Abdullah and Abu Hurayrah, and, in one of two narrations, from Ali and Ibn Abbas (may Allah be pleased with them). Ibn al-Mundhir said: That is not established from Abu Bakr and Uthman (may Allah be pleased with them both). The Shaykh says: From Abu Bakr it is munqati' (broken), and from Uthman it is mawsul (connected), and Allah knows best. Reports have come from the Prophet (peace and blessings be upon him) about a sa' of wheat, and reports have come about half a sa', but none of them is sound; I have explained the defect of each of them in al-Khilafiyyat. We have narrated in the hadith of Abu Sa'id al-Khudri, and in the established hadith from Ibn Umar, that equating two mudds of wheat, which is half a sa', with a sa' of barley happened after the Prophet (peace and blessings be upon him). And success is from Allah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7715