Abdullah bin Umar reported,
The Prophet ﷺ made obligatory the charity of Fitr, or he said of Ramadan, upon the male and the female, the free person and the slave, a saa of dates or a saa of barley. The people then considered half a saa of wheat as its equivalent.Ibn Umar 
used to give dates, but when dates became scarce for the people of Medina, he gave barley instead. Ibn Umar would give it on behalf of the young and the old, even on behalf of my sons. Ibn Umar would give it to those appointed to receive it, and the people would give it a day or two before the Fitr.
Azami 4283
Sahihعن ابن عمر، قال
فرض النبي ﷺ صدقة الفطر- أو قال رمضان- على الذكر والأنثى والحر والمملوك صاعا من تمر أو صاعا من شعير فعدل الناس به نصف صاع من بر. فكان ابن عمر رضي الله عنهما يعطي التمر فأعوز أهل المدينة من التمر فأعطى شعيرا، فكان ابن عمر يعطي عن الصغير والكبير حتى إن كان ليعطي عن بني، وكان ابن عمر رضي الله عنهما يعطيها الذين يقبلونها وكانوا يعطون قبل الفطر بيوم أو يومين.
Bukhari narrated it in zakat (1511), and Muslim narrated it in zakat (984:14), both of them through Ayub, from Nafi, from Ibn Umar, and he related it. The wording is Bukhari's, and Muslim does not have "Ibn Umar used to... etc." He also has it through the narration of Layth from Nafi, also in abbreviated form, and in it, Ibn Umar said, "So people made its equivalent two mudd of wheat."