Sahih Bukhari | Volume 2 | Hadith Number 579

Narrated by Ibn Umar
Allah's Apostle 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.)