Musnad Ahmad 1153:2

'Abbad ibn 'Abbad narrated to us, from Hisham ibn 'Urwah, from his father, from 'Aishah, who said: "The day of 'Ashura was a day that Quraysh used to fast during the Pre-Islamic period (Jahiliyyah), and the Messenger of Allah (peace and blessings of Allah be upon him) used to fast it. When he arrived in Medina, he fasted it and commanded that it be fasted. But when the obligation of the month of Ramadan was revealed, Ramadan was what he would fast, and he abandoned the day of 'Ashura. So whoever wished fasted it, and whoever wished did not fast it."