ʿĀshūrā’ is the 10th day in the month of Muḥarram, which is the first month in the Muslim lunar calendar. When the Prophet migrated to Madīnah he noticed that the Jews living there used to fast on this day and celebrate. Upon asking them why, they responded that this was the day when Allah saved Prophet Mūsā [Moses] and his followers from Firʿawn [the Pharaoh] by splitting the Red Sea for them and drowning the oppressive Egyptian army. They also noted that Prophet Mūsā himself fasted on this day as a sign of thanks to Allah. Upon hearing this, the Prophet Muhammad responded, “We are closer to Mūsā than you are”, indicating that Muslims should also be fasting on this day. The Prophet then ordered his followers to fast on the day of ʿĀshūrā’ as well.
When the Prophet Muhammad first prescribed it, fasting on the 10th of Muḥarram was mandatory because Muslims did not fast in Ramadan at that time. Then, when the month of Ramadan was prescribed for fasting, the obligation for the day of ʿĀshūrā’ was reduced to a recommendation. 
Some Companions chose not to fast on this day since the Prophet had given them the choice, but he himself continued to every year. In fact, ʿAbdullah ibn ʿAbbās related that he had never seen the Prophet so keen on fasting any other day than the day of ʿĀshūrā’. The Messenger of Allah emphasized fasting on this day to such an extent that he once said, “I hope that Allah would forgive the sins of the previous year for the one who fasts on the day of ʿĀshūrā’.”
Near the end of the Prophet’s life, he wanted Muslims to differentiate themselves from other religions so he stated, “Fast the day of ʿĀshūrā’ but differentiate yourself from the Jews by fasting the day before or after it.” After a detailed analysis of all the teachings of the Prophet relating to fasting on the day of ʿĀshūrā’, Imām Ibn Ḥajar al-ʿAsqalānī [d. 852 A.H./1449 C.E.] concluded that there are three different ways to fast this day, in order of ascending virtue:
- Fast only on the 10th day
- Fast on both the 9th and 10th day
- Fast on the 9th, 10th, and 11th
Whichever one of these a person chooses will have the reward for fasting on the day of ʿĀshūrā’. 
One of the misconceptions that has occurred in the minds of many Muslims is that the 10th of Muḥarram should be observed as a day of mourning because Husayn ibn ʿAlī, the righteous grandson of the Prophet, was killed on this day. It is true that the day he was killed was indeed sad and a great tragedy. However, many righteous people have been killed on other days, yet none of those dates have been taken as days of mourning. Ḥusayn’s own father, ʿAlī, was also killed unjustly in the month of Ramadan in the year 40 A.H. but no one has specified that day as a day of mourning to be observed by Muslims. Furthermore, the Messenger of Allah witnessed the slaying of many of his family members who were very dear to him such as his uncle Ḥamza, his adopted son Zayd ibn Ḥārithah, and his cousin Jaʿfar [ʿAlī’s brother], yet he never appointed a day of mourning for them. Therefore, although it may be beneficial to reflect on the martyrdom of Ḥusayn and the historical lessons that can be derived from such an incident, there is no basis for singling out his death as a day of mourning.
 Ṣaḥīḥ al-Bukhārī 3:43, #2004
 Ṣaḥīḥ al-Bukhārī 3:43, #2002
 Ṣaḥīḥ al-Bukhārī 3:43, #2006
 Ṣaḥīḥ Muslim 2:818, #1162; Muslim scholars have explained that this refers to minor sins only.
 Aḥmad ibn Ḥanbal, Musnad, 4:52, #2154; Ibn Ḥajar and other scholars deemed this narration to be weak because the narrator Ibn Abī Laylā did not have a very strong memory but Aḥmad Shākir declared it to be sufficient [ḥasan] because there are several reports from other narrators to this effect which give it some strength.
 Ibn Ḥajar al-ʿAsqalānī, Fatḥ al-Bārī Sharḥ Ṣaḥīḥ al-Bukhārī, 4:245