Qays bin al-Sakan (may Allah have mercy on him) narrates: I was sitting with Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) on the day of Ashura, and there was a bowl of tharid in front of him. Then al-Ash'ath bin Qays came in to him, and (Sayyiduna Abdullah (may Allah be pleased with him)) said: "O Abu Muhammad, will you not come near to the meal?" He said: "Have you people not fasted this day?" (Sayyiduna Abdullah (may Allah be pleased with him)) said: "This is a day on which we used to fast before Ramadan was revealed; then when Ramadan was revealed, we fasted it and left what was besides it."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10347