From Sahih Bukhari (Hadith 1592):
Narrated `Aisha:The people used to fast on 'Ashura (the tenth day of the month of Muharram) before the fasting of Ramadan was made obligatory. And on that day the Ka`ba used to be covered with a cover. When Allah made the fasting of the month of Ramadan compulsory, Allah's Messenger (ﷺ) said, "Whoever wishes to fast (on the day of 'Ashura') may do so; and whoever wishes to leave it can do so
Reference: Bukhari 1592