Umm al-Mu’minin Sayyida Aisha (may Allah be pleased with her) said: In the days of ignorance, the Quraysh used to fast on the day of Ashura. Then when the Messenger of Allah (peace and blessings be upon him) came to Madinah, he fasted on that day and ordered (the people) to fast. Then when the fasting of Ramadan was made obligatory, it became obligatory and Ashura was abandoned; whoever wished fasted and whoever wished left it.
Hadith Takhrijاس روایت کی سند صحیح اور حدیث متفق علیہ ہے۔ دیکھئے: [بخاري 1893، 2001، 2002] ، [مسلم 1125] ، [أبوداؤد 2442] ، [ترمذي 753] ، [أبويعلی 4638] ، [ابن حبان 3621] ، [الحميدي 202، وغيرهم]
Brief Explanation
(Commentary on Hadith 1800)
From all these ahadith, the virtue and importance of fasting on the day of Ashura is understood; therefore, it is recommended (mustahabb) to fast on the 10th of Muharram. Additionally, one should also fast either a day before or a day after.
The scholars (ulama) have mentioned three ways of doing this: to fast for three days—9th, 10th, and 11th; or to fast for two days—9th and 10th; or to fast for two days—10th and 11th.
The first method is the most virtuous. The scholars have considered it disliked (makruh) to fast only on the 10th of Muharram, because the Messenger of Allah (sallallahu alayhi wa sallam) said that if he lived until the next year, he would also fast on the 9th of Muharram.
There is another hadith that the Jews fast on the 10th of Muharram, so you should oppose them and fast on the 9th and 10th. Regarding the virtue of this fast, it is stated that the sins of the previous year are forgiven by it. May Allah Ta’ala grant everyone the ability to act upon this.