Sahih Bukhari
Obligatory Charity Tax After Ramadaan (Zakat ul Fitr)
Volume: 002 / Book Number: 025 / Hadith Number: 584
Narrated Abu Sa'id Al-Khudri:
In the life-time of the Prophet we used to give one Sa' of food or one Sa' of dates or one Sa' of barley or one Sa' of Raisins (dried grapes) as Sadaqat-ul-Fitr. And when Muawiya became the Caliph and the wheat was (available in abundance) he said, "I think (observe) that one Mudd (of wheat) equals two Mudds (of any of the above mentioned things).