Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrates: I never saw the Messenger of Allah (peace and blessings be upon him) seeking out the fast of any day, preferring it over other fasts, except this day, that is, the day of Ashura, and the month of Ramadan.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8398