From Sahih Bukhari (Hadith 1503):
Narrated Ibn `Umar:Allah's Messenger (ﷺ) enjoined the payment of one Sa' of dates or one Sa' of barley as Zakat-ul-Fitr on every Muslim slave or free, male or female, young or old, and he ordered that it be paid before the people went out to offer the `Id prayer. (One Sa' = 3 Kilograms approx)
Reference: Bukhari 1503