It was narrated from ‘Abdullah bin ‘Umar that the Day of ‘Ashura’ was mentioned in the presence of the Messenger of Allah (ﷺ). The Messenger of Allah (ﷺ) said: “That was a day when the people of the Ignorance used to fast. So whoever among you wants to fast may do so, and whoever does not want to may leave it.”
SahihSahihSahihBook 7 · Hadith 100