Sahih Muslim Book 6, Hadith Number 2522.
Chapter : Fasting on the day of ‘Ashura (1Oth of Muharram).
Abu Musa (Allah be pleased with him) reported: The day of ‘Ashura was one which the Jews respected and they treated it as Id. The Messenger of Allah (may peace be upon him) said: You also observe fast on this day.