Virtues of Fasting on Aashooraa'
Ibn 'Abbaas, may Allah be pleased with them both, said: "The prophet SAW, came to Madina and saw the Jews fasting on the day of Aashooraa. He said what is this? they said 'This is the righteous day; it is the day when Allah saved the children of Israel from their enemies, so Moosa fasted on this day.' He said: 'We have more right to Moosa than you,' so he fasted on that day and commanded [the Muslims] to fast on that day." (Reported by Al-Bukhari 1865).
Ibn 'Abbaas may Allah be pleased with them both said, "I never saw the Messenger of Allah SAW so keen to fast any day and give it priority over any other than this day, the day of 'Aashooraa, and this month meaning Ramadan." (Reported by Al-Bukhari 1867).
The Prophet SAW said, "for fasting the day of 'Aashooraa', I hope that Allah will accept it as expiation for the year that went before." (Reported by Muslim, 1976).
This is from the bounty of Allah towards us: for fasting one day He gives us expiation for the sins of a whole year. And Allah is the Owner of Great Bounty.