Sahih Al Bukhari Hadith Number 1512

Narrated Ibn `Umar: Allah’s Messenger (peace be upon him) 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.

24  Obligatory Charity Tax (Zakat)
Hadith number 1512
Volume no 2
Chapter: 78 (Sadaqat-ul-Fitr is obligatory on the young and the old)