Sahih Muslim Book 6, Hadith Number 2526.

Chapter : On which day the fast is to be observed in Muharram.

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.