From Sahih Muslim (Hadith 2645):
Abdullah b. 'Umar (Allah be pleased with both of them) reported that he heard the Messenger of Allah (ﷺ) say about the day of Ashura:It is a day on which the people of pre-Islamic days observed fast. So he who liked to fast on this day should do so, and he who liked to abandon it should abandon it. 'Abdullah (Allah be pleased with him) did not observe fast except when it coincided (with the days when he was in the habit of observing voluntary fasts during every month)
Reference: Muslim 2645