Abu Sa'id al-Khudri (may Allah be pleased with him) narrates that during the time of the Prophet ﷺ, we used to give one sa' of food, or one sa' of raisins, or one sa' of cheese, or one sa' of dates, or one sa' of barley as zakat al-fitr. We continued to give this until Mu'awiyah (may Allah be pleased with him) came for Hajj or 'Umrah, delivered a sermon to the people, and among what he said to them was: "In my opinion, two mudd of wheat from Syria are equal to one sa' of dates." Then the people adopted this method.
Hadith Referenceمسند الإمام الشافعي / كتاب الزكاة / 668
Hadith Takhrijاخرجه مسلم ،الزكاة، باب زكاة الفطر على المسلمين من التمر والشعير (985)۔