Muwatta Malik #686

Yahya related to me from Malik from Hisham ibn Urwa from his father that A'isha, the wife of the Prophet, may Allah bless him and grant him peace, said, "The day of Ashura was a day the Quraysh used to fast in the jahiliyya, and the Messenger of Allah, may Allah bless him and grant him peace, used also to fast it during the jahiliyya. Then when the Messenger of Allah, may Allah bless him and grant him peace, came to Madina he fasted it and ordered that it be fasted. Then Ramadan was made obligatory, and that became the fard instead of Ashura, but whoever wanted to, fasted it, and whoever did not want to, did not fast it."