Sahih Muslim
The Book of Fasting (Kitab Al-Sawm)
Book Number: 006 / Hadith Number: 2528
Ibn 'Abbas reported that when the Messenger of Allah (may peace be upon him) 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 (may peace be upon him) said: When the next year comes, God willing, we would observe fast on the 9th But the Messenger of Allah (may peace be upon him) died before the advent of the next year.