Yahya related to me from Malik from Hisham ibn Urwa from hisfather that A'isha, the wife of the Prophet, may Allah bless him andgrant him peace, said, "The day of Ashura was a day the Quraysh usedto fast in the jahiliyya, and the Messenger of Allah, may Allah blesshim and grant him peace, used also to fast it during the jahiliyya.Then when the Messenger of Allah, may Allah bless him and grant himpeace, came to Madina he fasted it and ordered that it be fasted. ThenRamadan was made obligatory, and that became the fard instead ofAshura, but whoever wanted to, fasted it, and whoever did not want to,did not fast it."
USC-MSA web (English) reference: Book 18, Hadith 33
Arabic reference: Book 18, Hadith 667