Musnad Abu Ya'la al-Mawsili 152:107

Abu Khaythama narrated to us, Jarir narrated to us, from al-A'mash, from 'Umarah ibn 'Umayr, from 'Abd al-Rahman ibn Yazid, who said: Al-Ash'ath ibn Qays entered upon 'Abdullah while he was eating lunch. He said, "Come near." Al-Ash'ath said, "Is today not the day of 'Ashura?" 'Abdullah said, "And what makes you know what 'Ashura is? The Messenger of Allah (may Allah bless him and grant him peace) used to fast it, but when Ramadan was revealed, he abandoned it."