From Sahih Bukhari (Hadith 1512):
Narrated Ibn `Umar:Allah's Messenger (ﷺ) has made Sadaqat-ul-Fitr obligatory, (and it was), either one Sa' of barley or one Sa' of dates (and its payment was obligatory) on young and old people, and on free men as well as on slaves
Reference: Bukhari 1512