The Prophet (ﷺ) ordered (Muslims) to give one Sa' of dates or one Sa' of barley as Zakat-ul-Fitr. The people rewarded two Mudds of wheat as equal to that.
Sahih al-Bukhari Book 24, Hadith 107
← Previous Hadith We used to give one Sa' of meal or one Sa' of barley or one Sa' of dates, or one Sa' of cottage cheese or one Sa' of Raisins (dried grapes) as Zakat-ul-Fitr.

Sahih al-Bukhari Book 24, Hadith 106 Next Hadith → In the lifetime of the Prophet (ﷺ) we used to give one Sa' of food or one Sa' of dates or one Sa' of barley or one Sa' of Raisins (dried grapes) as Sadaqat-ul-Fitr. And when Muawiya became the Caliph and the wheat was (available in abundance) he said, "I think (observe) that one Mudd (of wheat) equals two Mudds (of any of the above mentioned things).

Sahih al-Bukhari Book 24, Hadith 108