Sahih Muslim | Book 6 | Hadith Number 2524

Narrated by Ibn Abbas
Ibn Abbas was asked about observing of fast on the day of Ashura, whereupon he said: I do not know Allah's Messenger (may peace be upon him) singling out any day's fast and considering it more excellent than another, except this day (the day of Ashura) and this month, meaning the month of Ramadan.