From Sahih Muslim (Hadith 2642):
Abdullah b. 'Umar (Allah be pleased with them) reported that (the Arabs of) pre-Islamic days used to observe fast on the day of Ashura and the Messenger of Allah (ﷺ) observed it and the Muslims too (observed it) before fasting in Ramadan became obligatory. But when it became obligatory, the Messenger of Allah (ﷺ) said:'Ashura is one of the days of Allah, so he who wished should observe fast and he who wished otherwise should abandon it
Reference: Muslim 2642