Al-Lu'lu Wal-Marjan 692 — The Book of As-Siyâm (the Fasting)

Ibn 'Abbas Narrated from:

The Prophet ﷺ came to Al-Madîna and saw the Jews observing Saum (fast) on the day of ' Ashûra. He asked them about that. They replied, "This is a good day, the day on which Allâh rescued Banî Israel from their enemy. So, Mûsa (Moses) observed Saum on this day." The Prophet ﷺ said, "We have more claim over Mûsa than you." So, the Prophet ﷺ observed Saum on that day and ordered (the Muslims) to observe Saum (on that day). ( Sahîh Al-Bukhârî, Hadîth No. 222, Vol. 3)