It wasnarrated from 'Abdullah bin 'Umar that the Day of 'Ashura'wasmentioned in the presence of the Messenger of Allah (ﷺ). TheMessenger of Allah (ﷺ) said:
"That was a day when the people oftheIgnorance used to fast. So whoever among you wants to fast may doso,and whoever does not want to may leave it."
English reference: Vol. 1, Book 7, Hadith 1737
Arabic reference: Book 7, Hadith 1809