(75) باب صَاعٍ مِنْ زَبِيبٍ
Narrated Abu Sa`id Al-Khudri:
In the lifetime of the Prophet (ﷺ) 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).
Reference: Sahih al-Bukhari 1508
In-book reference: Book 24, Hadith 109
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 2, Book 25, Hadith 584