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." (FR)
English reference (FR): Vol. 1, Book 7, Hadith 1737 (FR)
Arabic reference (FR): Book 7, Hadith 1809 (FR)