Sahih Al Bukhari Hadith Number 1506

Narrated Abu Sa`id Al-Khudri: We used to give one Sa’ of meal or one Sa’ of barley or one Sa’ of dates, or one Sa’ of cottage cheese or one Sa’ of Raisins (dried grapes) as Zakat-ul-Fitr.

24  Obligatory Charity Tax (Zakat)
Hadith number 1506
Volume no 2
Chapter: 73 (Sadaqat-ul-Fitr is one Sa’ of meal (per head)