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." (JA)
English reference (JA): Vol. 3, Book 11, Hadith 2112 (JA)
Arabic reference (JA): Book 11, Hadith 2190 (JA)