The Historical Importance and Observance of ‘Āshūrā in Islam
‘A’ishah (may ALLĀH be pleased with her) narrated,
"The Day of 'Ashūrā was fasted by the Quraysh in the period before Islām and ALLAH'S Messenger (ﷺ) used to fast it before Islām. When he came to Madīnah he fasted and ordered it to be fasted. When Ramadān was obligated he gave up the Day of 'Āshūrā, so whoever wanted fasted it and whoever wanted left it." {Al-Bukhārī #2002 and Muslim #1125}.This hadith is evidence that the people of jāhiliiyah used to know about ‘Āshūrā, it was a famous day among them, and they used to fast it. The Prophet (ﷺ) used to fast it also and continued fasting it before migrating. He (ﷺ) did not command the people to fast it (after Ramadān was legislated). This is evidence for the sacredness of this day and the greatness of its status among the Arabs in the period before Islām, before the sending of the Prophet (ﷺ). Due to this they used to cover the Ka’bah on this day as is found in the hadith of ‘A’ishah (may ALLĀH be pleased with her) who said,
"They used to fast 'Āshūrā before Ramadān was obligated and it was the day they covered the Ka'bah…" {Al-Bukhārī #1952}Al-Qurtubi said, "The hadith of 'A'ishah shows that the legislation and status of fasting on this day was known to them. Perhaps they based fasting it on the legislation of Ibrāhīm and Ismā'īl (peace be upon them) because they used to ascribe themselves to these Prophets and based many of their rulings for Hajj and other than that on them…" {Al-Mufhim, 3/190}What is derived from the gathering of evidences is that fasting ‘Āshūrā was obligatory in the beginning after the migration of the Prophet (ﷺ) to al-Madīnah according to the correct opinion of the people of knowledge. The command to fast it is established by the hadith of Salamah ibn al-Akwa’ (may ALLĀH be pleased with him). He said,
"The Prophet (ﷺ) ordered a man from the tribe of Aslam to announce to the people: 'Whoever was eating then let him fast the rest of the day and whoever did not eat then let him fast for today is the Day of 'Ashūrā." {al-Bukhārī #2007 and Muslim #1135}When Ramadān was made obligatory in the second year of the hijrah, the obligation to fast ‘Āshūrā was abrogated but it remained recommended. The command to fast the Day of ‘Ashūrā was only for one year and that was the second year of hijrah when it was obligated in the beginning of that year. Then Ramadān was obligated after half of that year passed. At the end of his life in the tenth year, the Prophet (ﷺ )determined not to fast it by itself but to fast the 9th day before it as will be shown later if ALLĀH wills. This is an instance of differing with the People of the Book in the way that they fast.
{Taken from: Ahadith about the Tens Days of Dhul-Hijjah and the Days of Tashriq, Rulings and Manners || Page 66-68}