Allah's Messenger (ﷺ) ordered (the Muslims) to fast on the day of 'Ashura', and when fasting in the month of Ramadan was prescribed, it became optional for one to fast on that day ('Ashura') or not.
Sahih al-Bukhari Book 30, Hadith 106
← Previous Hadith The Prophet (ﷺ) said, "Whoever wishes may fast on the day of 'Ashura'."

Sahih al-Bukhari Book 30, Hadith 105 Next Hadith → Quraish used to fast on the day of 'Ashura' in the Pre-Islamic period, and Allah's Messenger (ﷺ) too, used to fast on that day. When he came to Medina, he fasted on that day and ordered others to fast, too. Later when the fasting of the month of Ramadan was prescribed, he gave up fasting on the day of 'Ashura' and it became optional for one to fast on it or not.

Sahih al-Bukhari Book 30, Hadith 107