Al-Hasan (may Allah have mercy on him) says that Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) addressed us in Basrah at the end of Ramadan and said: "Pay the zakah of your fasts." It was as if the people did not know about it. Then he said: "Who is here from the people of Madinah? Let them teach their brothers, for these people do not know that the Messenger of Allah (peace and blessings be upon him) made this zakah obligatory: 'Upon every young and old, male and female, free and slave: a sa' of dates or barley, or half a sa' of wheat.'" When Sayyiduna Ali (may Allah be pleased with him) came and saw that barley was cheap, he said: "If you made it a sa' of everything, that would be better." The narrator said: And al-Hasan held that it is due upon whoever fasted.
This is how he narrated it, saying "he addressed us"; Muhammad bin al-Muthanna narrated it from Sahl bin Yusuf and said "he addressed (the people)," and that is more correct.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7712