Abu Sa'd said: I went to Sayyiduna Ibn Mas'ud (may Allah be pleased with him) on the day of Ashura while he was eating. Then a man came in, and Sayyiduna Ibn Mas'ud (may Allah be pleased with him) said: "Eat." He said: "I am fasting." He said: "Eat; this is a day that used to be fasted before (the fasting of) Ramadan was revealed. Then Ramadan was revealed, and this day was left."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10475