Sahih Bukhari
Obligatory Charity Tax After Ramadaan (Zakat ul Fitr)
Volume: 002 / Book Number: 025 / Hadith Number: 588
Narrated Ibn 'Umar:
Allah's Apostle has made Sadaqatul-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.