The Messenger of Allah ﷺ ordered the zakat al-fitr to be one saa of dates or one saa of barley. Abdullah said, "The people later considered two mudds of wheat to be its equivalent."
حدثنا محمد بن رمح المصري أنبأنا الليث بن سعد عن نافع
أن رسول الله ﷺ أمر بزكاة الفطر صاعا من تمر أو صاعا من شعير قال عبد الله فجعل الناس عدله مدين من حنطة
Bukhari narrated it in the Book of Zakat, Chapter on Sadaqat al-Fitr Being One Saa of Dates, Hadith 1507, from the hadith of Layth, with this wording. Muslim narrated it in the Book of Zakat, Chapter on Zakat al-Fitr Being Obligatory upon Muslims from Dates and Barley, Hadith 984, from Muhammad bin Rumh and others.