Abd al-Rahman, the son of Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him), narrates that al-Ash'ath bin Qays came to his father while he was eating on the day of Ashura, and Sayyiduna Abdullah (may Allah be pleased with him) said: "This is merely a day on which we used to fast before (the obligation of) Ramadan."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10331