Sahih Bukhari
Obligatory Charity Tax After Ramadaan (Zakat ul Fitr)
Volume: 002 / Book Number: 025 / Hadith Number: 583
Narrated 'Abdullah bin 'Umar
The Prophet ordered (Muslims) to give one Sa' of dates or one Sa' of barley as Zakat-ul-Fitr. The people rewarded two Mudds of wheat as equal to that.