When the Prophet (ﷺ) on the day of ‘Ashurah and commanded us to fast on it, they (i.e. Companions) said: Messenger of Allah, this is a day which is considered great by Jews and Christians ? The Messenger of Allah (ﷺ) said: When the next year comes, we shall fast on the 9th of Muharram. But the next year the Messenger of Allah (ﷺ) breathed his last.
“I met up with Ibn Abbas while he was reclining on his Rida at the Zamzam (well). So I said: ‘Inform me about the Day of Ashura. On which day is it fasted?’ He said: ‘When you see the crescent of Al-Muharram, then count, then fast on the morning of the ninth day.'” He said: ‘I said: Is this how Muhammad fasted it?’ He said: ‘Yes.'”