Musnad Ahmad 26:798

Muhammad ibn Ubayd narrated to us, al-A’mash narrated to us, from ‘Umarah, from ‘Abd al-Rahman ibn Yazid, who said: Al-Ash’ath ibn Qays entered upon ‘Abdullah while he was eating lunch. He said, "O Abu Muhammad, come near to the lunch." He said, "Is today not the day of Ashura?" He said, "And what is it?" He said, "It is only a day that the Messenger of Allah (peace and blessings of Allah be upon him) used to fast before Ramadan. When the month of Ramadan was revealed, he abandoned it."