Sahih Muslim Book 6, Hadith Number 2529.
Chapter : On which day the fast is to be observed in Muharram.
Abdullah b ‘Abbas reported that the Messenger of Allah (may peace be upon him) had said: 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.”