Musnad Ahmad 1153:1276

Yahya ibn Zakariyya narrated to us, Hisham ibn Urwah narrated to us, from his father, from Aisha, who said: "The day of Ashura was a day that the Messenger of Allah (peace and blessings of Allah be upon him) used to fast during the Pre-Islamic Era (Jahiliyyah), and the Quraysh used to fast it during the Pre-Islamic Era. When the Prophet (peace and blessings of Allah be upon him) arrived in Medina, he fasted it and commanded that it be fasted. But when Ramadan was revealed, Ramadan became the obligatory fast, and Ashura was left."