From Sahih Bukhari (Hadith 1893):
Narrated `Aisha:(The tribe of) Quraish used to fast on the day of Ashura' in the Pre-Islamic period, and then Allah's Apostle ordered (Muslims) to fast on it till the fasting in the month of Ramadan was prescribed; whereupon the Prophet (ﷺ) said, "He who wants to fast (on 'Ashura') may fast, and he who does not want to fast may not fast
Reference: Bukhari 1893