The day of 'Ashura' was considered as `Id day by the Jews. So the Prophet (ﷺ) ordered, "I recommend you (Muslims) to fast on this day."
Sahih al-Bukhari Book 30, Hadith 110
← Previous Hadith The Prophet (ﷺ) came to Medina and saw the Jews fasting on the day of Ashura. He asked them about that. They replied, "This is a good day, the day on which Allah rescued Bani Israel from their enemy. So, Moses fasted this day." The Prophet (ﷺ) said, "We have more claim over Moses than you." So, the Prophet fasted on that day and ordered (the Muslims) to fast (on that day).

Sahih al-Bukhari Book 30, Hadith 109 Next Hadith → I never saw the Prophet (ﷺ) seeking to fast on a day that he favored more than another except this day, the day of 'Ashura', and this month, meaning the month of Ramadan.

Sahih al-Bukhari Book 30, Hadith 111