Narrated Ibn 'Umar:
Fasting was observed on the day of 'Ashura' (i.e. 10th of Muharram) by the people of the Pre-lslamic Period. But when (the order of compulsory fasting) in the month of Ramadan was revealed, the Prophet ﷺ said, "It is up to one to fast on it (i.e. day of 'Ashura') or not."
Sahih al-Bukhari, Vol. 6, Book of Prophetic Commentary on the Qur'an, Hadith 28
Sahih al-Bukhari, Book of Prophetic Commentary on the Qur'an, Hadith 28