Musnad Ahmad 25:1432

Yazid narrated to us, he said: Humayd informed us, from al-Hasan, who said: Ibn Abbas addressed the people at the end of Ramadan and said: "O people of Basra, pay the Zakat of your fast." He said: The people began to look at one another. He then said: "Who is here from the people of Medina? Stand up and teach your brothers, for they do not know that the Messenger of Allah, may Allah bless him and grant him peace, obligated the charity of Ramadan as half a sa' of wheat, or a sa' of barley, or a sa' of dates, upon the slave and the free, the male and the female."