Musnad Ahmad 5:505

Abdullah narrated to us, Abu Kurayb al-Hamdani narrated to me, Mu'awiyah ibn Hisham narrated to us, from Sufyan al-Thawri, from Jabir, from Sa'd ibn 'Ubaydah, from Abu 'Abd al-Rahman, from 'Ali: "That the Messenger of Allah, peace and blessings of Allah be upon him, used to fast on the day of 'Ashura and command it."