Sayyiduna Abu Sa'id (may Allah be pleased with him) says that in the time of the Messenger of Allah (peace and blessings be upon him) we used to give, as Sadaqat al-Fitr, one sa' of wheat, barley or cheese (aqit).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7729