All praise is due to Allah. May Allah’s salah and salam be upon the Messenger of Allah, his family, his Companions, and all those who follow his guidance.

To proceed,
It has been authentically reported that the Prophet (may Allah’s salah and salam be upon him) would actively seek out the Day of ‘Ashura’ in order to fast on it, and that he would also encourage the people to do so. This is because it is the day on which Allah saved Musa and his people and destroyed Fir‘aun and his people. Therefore, it is recommended for every male and female Muslim to fast on this day out of gratitude to Allah, the Most High. In relation to this, when ibn ‘Abbas (may Allah be pleased with him and his father) was asked about fasting on the Day of ‘Ashura’, he replied, “I never saw the Messenger of Allah (may Allah’s salah and salam be upon him) actively seek to fast on any day, considering it superior to other days, except on the day of ‘Ashura’ and during the month of Ramadan.” This was reported by al-Bukhari and Muslim.

Likewise, regarding its merits, Abu Qatadah (may Allah be pleased with him) narrated that when a man asked the Prophet (may Allah’s salah and salam be upon him) about fasting on the Day of ʿAshuraʾ, he replied, “I hope that, through it, Allah will expiate the sins of the previous year.” This was reported by Muslim.

It is also recommended to fast the day before ‘Ashura’ in addition to the day itself, because the Prophet (may Allah’s salah and salam be upon him) resolved in the latter part of his life not to fast it alone; rather, he wanted to add another day to it, thereby differing from the People of the Book in his fasting. In support of this, ibn ʿAbbas (may Allah be pleased with him) said, "When the Messenger of Allah (may Allah’s salah and salam be upon him) fasted on the day of ‘Ashura’ and commanded its fasting, the people said, “O Messenger of Allah, this is a day revered by the Jews and the Christians.” The Prophet (may Allah’s salah and salam be upon him) replied, “If I remain alive until next year, I will most certainly fast the ninth.” However, the Messenger of Allah passed away before the following year came.

In another narration of ibn ‘Abbas (may Allah be pleased with him), the Prophet said, “If I remain alive until next year, I will most certainly fast the ninth.”—meaning along with ‘Ashura’.

Ibn al-Qayyim mentioned in his book, Zad al-Ma‘ad, that fasting on the Day of ‘Ashura’ comprises three distinct levels. The first and most complete of these is fasting in addition, the day before and the day after ‘Ashura’, thus, fasting the ninth, tenth, and eleventh. The second is fasting the ninth and tenth, as supported by the majority of related narrations, and the third is fasting the tenth day alone.