Sahih Muslim | Book 6 | Chapter 20

Narrated by Hakam b. al-'Araj
Hakam b. al-'Araj reported: I went to Ibn 'Abbas (Allah be Pleased with both of them) and he was reclining using his mantle as a pillow near the fountain of Zamzam. I said to him: Tell me about fasting on Ashura. He said: When you see the new moon of Muharram then count the (days) and observe fast on the 9th. I said to him: Is it how the Messenger of Allah (may peace be upon him) observed the fast? He said: Yes.

Narrated by Hakam b. 'Araj
Hakam b. 'Araj reported: I asked Ibn Abbas (Allah be pleased with them) as he was reclining using his cloak as a pillow near Zamzam about fasting on 'Ashura. The rest of the hadith is the same.

Narrated by Ibn 'Abbas
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.

Narrated by Abdullah b 'Abbas
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.