The Messenger of Allah ﷺ made the Ramadan charity obligatory upon every male and female, free person and slave, in the amount of one saa of dates or one saa of barley. The people later considered half a saa of wheat as its equivalent.Ayub said that Nafi reported, "Ibn Umar used to give dates, except for one year when dates were scarce, so he gave barley."
حدثنا إسماعيل أخبرنا أيوب عن نافع
فرض رسول الله ﷺ صدقة رمضان على الذكر والأنثى والحر والمملوك صاع تمر أو صاع شعير قال فعدل الناس به بعد نصف صاع بر قال أيوب وقال نافع كان ابن عمر يعطي التمر إلا عاما واحدا أعوز التمر فأعطى الشعير
Its isnad is sahih. It was narrated by the group, as in al-Muntaqa 2084, except that the statement "then people switched to half a saa of wheat" to the end of the hadith was narrated by Abu Dawud 28:2 through the route of Hammad from Ayub. Munziri said, Bukhari, Muslim, Tirmidhi, and Nasai narrated it. See 3291.