Musnad Ahmad 27:39

Isma'il narrated to us, Ayyub informed us, from Nafi', from Ibn Umar, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, obligated the charity of Ramadan upon the male, the female, the free, and the slave: a sa' of dates or a sa' of barley." He said: "Then the people equated it afterward to half a sa' of wheat." Ayyub said, and Nafi' said: "Ibn Umar used to give dates, except for one year when dates were scarce, so he gave barley."