And what is obligatory according to this principle is to give charity of a measure (sa’) of wheat in some times and some lands.

Sayyiduna Abu Sa'id Khudri (may Allah be pleased with him) narrates that during the blessed era of the Messenger of Allah (peace and blessings be upon him), we used to give one sa' of dates, one sa' of barley, or one sa' of cheese (as zakat al-fitr). This practice continued after him until the era of Sayyiduna Mu'awiyah (may Allah be pleased with him), when he said: "In my opinion, one sa' of wheat from Syria is equal to two sa' of dates." So the people began to give (half a sa' of wheat as zakat al-fitr) according to this opinion.