Musnad Abu Ya'la al-Mawsili 150:134

'Abd al-A'la narrated to us, Yahya bin Sa'id narrated to us, from Hisham bin 'Urwah, my father informed me, from 'Aishah, she said: The day of 'Ashura was a day that Quraysh used to fast during the Pre-Islamic period (Jahiliyyah), and the Messenger of Allah (ﷺ) used to fast it. When he arrived in Medina, he fasted it and commanded that it be fasted. When the fasting of Ramadan was revealed, Ramadan became the obligatory duty, and 'Ashura was abandoned. So, whoever wished fasted it, and whoever wished did not fast it.