Musannaf Ibn Abi Shayba 4:491

Ibn Numayr narrated to us, he said: Hisham ibn Urwah narrated to us, from his father, from Aisha, who said: Ashura was a day that Quraysh used to fast during the Pre-Islamic period (Jahiliyyah). When the Messenger of Allah (peace be upon him) arrived in Medina, he fasted it and commanded that it be fasted. Then, when Ramadan was made obligatory, it became the obligation, and Ashura was abandoned. So whoever wished fasted it, and whoever wished left it.