He who wishes to fast should do so, and he who wishes to break it may do so.
Sahih Muslim Book 13, Hadith 147
← Previous Hadith 'A'isha (Allah be pleased with her) reported that the Messenger of Allah (ﷺ) had ordered to observe fast (on 'Ashura) before the fasting in Ramadan was made obligatory. But when it became obligatory, then he who wished fasted on the day of Ashura, and he who wished did not observe it (on that day).

Sahih Muslim Book 13, Hadith 146 Next Hadith → 'Ashura is one of the days of Allah, so he who wished should observe fast and he who wished otherwise should abandon it.

Sahih Muslim Book 13, Hadith 148