صيام يوم عاشوراء، إني أحتسب على الله أن يكفر السنة التي قبله
“For fasting the day of `Ashura I hope Allah will expiate [sins] thereby for the year that came before it.” (Muslim)
The reason for fasting `Ashura is because this was the day that Allah saved Musa (`alayhi assalam) and the children of Israel from Pharoah, and the Jews at the time of the Prophet ﷺ would fast this day for this reason too (Bukhari).
Ibn `Abbas also quoted the Prophet ﷺ as saying, “If I live next year, I shall also fast on the 9th day,” (Musnad Ahmad). This is in order to distinguish the Muslim practices from the Jewish ones (Tirmidhi). An-Nawawi also mentioned that one of the reasons for fasting the 9th day is as a precaution because of the possibility of error when sighting the new moon. However, it is still permissible to only fast on the 10th day.
It should not be forced on one to fast it, because the Prophet ﷺ said: “Concerning the day of `Ashura, it is not obligatory upon you to fast on it as I do. Whoever wishes may fast and whoever does not wish to is not obliged to do so,” (Bukhari and Muslim). Although who of us doesn’t want all their minor sins expiated?
Please remember to pray for our brothers and sisters all over the world. May Allah accept the fasts and du`a’ of all those who choose to fast. Ameen.
Source: Virtual Mosque