Majma al-Zawaid — Hadith 5128

Abdullah bin Abi Sa'd narrates: We came to A'idh bin Amr on the day of Ashura, and he said: "O boy, milk (an animal) for them." The boy got up, went to a she-goat and milked it, then he came to them. A'idh said to the person on his right: "Drink." He said: "I am fasting." A'idh said: "May Allah Almighty accept it from us and from you." Then he said to the second person, and he too said: "I am fasting," so A'idh said similar words. He said to the third, and he said similar words. He asked: "Are all of you fasting? Soon you will start regarding this day like Ramadan. We used to fast on this day before Ramadan was made obligatory. When Ramadan was made obligatory upon us, the fast of Ramadan abrogated the fast of this day. So today is a day of voluntary fasting, not obligatory; whoever wishes may fast, and whoever wishes may leave the fast." When the people heard this, all of them broke their fast.
Al-Tabarani recorded it in al-Mu'jam al-Kabir. Its chain contains a narrator, Hashraj bin Abdullah; I have not found anyone who gives his biography.