It was narrated that Ibn Abbas said:
"The Messenger of Allah (ﷺ) offered expiation of a Sa' of dates, and he enjoined the people to do likewise. Whoever does not have that (must give) half a Sa' of wheat." (IT)
English reference (IT): Vol. 3, Book 11, Hadith 2112 (IT)
Arabic reference (IT): Book 11, Hadith 2190 (IT)