Sahih Muslim
The Book of Fasting (Kitab Al-Sawm)
Book Number: 006 / Hadith Number: 2529
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."