Sahih Muslim
The Book of Fasting (Kitab Al-Sawm)
Book Number: 006 / Hadith Number: 2524
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.