basically because:
Hadhrat Abdullâh bin Abbâs Radhi-Allâhu ‘anhu reports that when Rasulullâh Sallallâhu ‘alayhi wasallam came to Madinah, he noticed that the Jews fasted on the day of Âshura. When he asked them the reason for this, they replied, “This is a great day. It was on this day that Allâh Ta’âla delivered the Bani Isrâ’eel from their enemy. On this day Allâh Ta’âla granted victory to Hadhrat Moosa Alayhis-Salâm and the Bani Isrâ’eel. We fast because of the distinction of this day.” Rasulullâh Sallallâhu ‘alayhi wasallam then said to the Jews, “We are closer to Hadhrat Moosa Alayhis-Salâm than you.” Rasulullâh Sallallâhu ‘alayhi wasallam also observed this fast and instructed the Muslims to follow suit.[8]
