Ibn Abbaas (may Allaah be pleased with him) said: The Prophet (peace and blessings of Allaah be upon him) came to Madeenah and saw the Jews fasting on the day of Aashooraa. He said, What is this? They said, This is a 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-Bukhaari, 1865).