Musnad Ahmad 26:474

Ya'la and Ibn Abi Za'idah narrated to us, they said: Al-A'mash narrated to us, from 'Umarah, from 'Abd al-Rahman ibn Yazid, who said: Al-Ash'ath ibn Qays entered upon 'Abdullah on the day of 'Ashura while he was eating lunch. He said: "O Abu Muhammad, come near for lunch." He said: "Is it not the day of 'Ashura?" He said: "And do you know what the day of 'Ashura is? The Messenger of Allah, peace and blessings of Allah be upon him, used to fast it before Ramadan was revealed. But when Ramadan was revealed, he abandoned it."