Ibn 'Umar (Allah be pleased with them) said that the day of 'Ashura was mentioned before the Messenger of Allah (may peace he upon him). Thereupon the Messenger of Allah , (ﷺ) said
That was a day on which the people of pre-Islamic days used to observe fast. So he who amongst you likes to observe fast should do so , and he who does not like it should abandon it.