Sahih Muslim Book 6, Hadith Number 2524.
Chapter : Fasting on the day of ‘Ashura (1Oth of Muharram).
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.