From Sahih Muslim (Hadith 2666):
Ibn 'Abbas reported that when the Messenger of Allah (ﷺ) fasted on the day of 'Ashura and commanded that it should he observed as a fast, they (his Companions) said to him:Messenger of Allah, it is a day which the Jews and Christians hold in high esteem. Thereupon the Messenger of Allah (ﷺ) said: When the next year comes, God willing, we would observe fast on the 9th But the Messenger of Allah (ﷺ) died before the advent of the next year
Reference: Muslim 2666