If I live till the next (year), I would definitely observe fast on the 9th, and the narration transmitted by Abu Bakr is:" He meant the day of Ashura."
Sahih Muslim Book 13, Hadith 173
← Previous Hadith 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.

Sahih Muslim Book 13, Hadith 172 Next Hadith → Salama b. al-Akwa' (Allah be pleased with him) reported that the Messenger of Allah (ﷺ) sent a person of the tribe of Aslam on the day of 'Ashura and commanded him to declare to the people to observe fast in case they had not observed it, and to complete fast till evening if they had taken food

Sahih Muslim Book 13, Hadith 174