Abu Dawud Book 3, Hadith Number 1618.

Narated By Abdullah ibn Abbas : Al-Hasan said: Ibn Abbas preached towards the end of Ramadan on the pulpit (in the mosque) of al-Basrah. He said: Bring forth the sadaqah relating to your fast. The people, as it were, could not understand. Which of the people of Medina are present here? Stand for your brethren, and teach them, for they do not know.

The Apostle of Allah (PBUH) prescribed this sadaqah as one sa’ of dried dates or barley, or half a sa’ of wheat payable by every freeman or slave, male or female, young or old. When Ali came (to Basrah), he found that price had come down. He said: Allah has given prosperity to you, so give one sa’ of everything (as sadaqah).

The narrator Humayd said: Al-Hasan maintained that the sadaqah at the end of Ramadan was due on a person who fasted.