Musnad Ahmad 1007:72

'Abd al-Razzaq narrated to us, Sufyan informed us, from Mansur, from Mujahid, from Harmalah ibn Iyas al-Shaybani, from Abu Qatadah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Fasting the day of 'Arafah is an expiation for two years: a past year and a coming year. And fasting the day of 'Ashura is an expiation for a year."