From Sahih Bukhari (Hadith 1510):
Narrated Abu Sa`id Al-Khudri:In the lifetime of Allah's Messenger (ﷺ) , we used to give one Sa' of food (edible things) as Sadaqat-ul-Fitr (to the poor). Our food used to be either of barley, raisins (dried grapes), cottage cheese or dates
Reference: Bukhari 1510