The Prophet (ﷺ) came to Medina and saw the Jews fasting on the day of Ashura. He asked them aboutthat. They replied, "This is a good day, the day on which Allah rescued Bani Israel from their enemy.So, Moses fasted this day." The Prophet (ﷺ) said, "We have more claim over Moses than you." So, theProphet fasted on that day and ordered (the Muslims) to fast (on that day).