Musnad Ahmad 1007:14

Sufyan narrated to us, he said: We heard it from Dawud ibn Shabur, from Abu Qaza’ah, from Abu al-Khalil, from Abu Harmalah, from Abu Qatadah, who said: "Fasting the day of Arafah expiates the year [before it] and the one that follows it, and fasting the day of Ashura expiates a year." Abdullah said: My father said: Sufyan did not attribute it to the Prophet (marfu’) for us, though it is attributed (marfu’).