The Prophet (ﷺ) said, "Whoever wishes may fast on the day of 'Ashura'."
Sahih al-Bukhari Book 30, Hadith 105
← Previous Hadith Fasting for those who perform ,Hajj-at-Tamattu` (in lieu of the Hadi which they cannot afford) may be performed up to the day of `Arafat. And if one does not get a Hadi and has not fasted (before the `Id) then one should fast of the days of Mina. (11, 12 and 13th of Dhul Hajja).

Sahih al-Bukhari Book 30, Hadith 104 Next Hadith → 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