Sayyiduna Ibn Umar (may Allah be pleased with them both) narrates that the Messenger of Allah (peace and blessings be upon him) said: "Sadaqat al-Fitr is one sa’ of barley, or one sa’ of dates, or one sa’ of Hijazi barley."
Hadith Referenceصحيح ابن خزيمه / جماع أبواب صدقة الفطر فى رمضان / 2416