Musnad Ahmad 1007:1

Hashim ibn Bashir narrated to us, Mansur—meaning Ibn Zadhan—informed us, from Qatadah, from Abdullah ibn Ma'bad al-Zamani, from Abu Qatadah, that the Messenger of Allah, peace and blessings of Allah be upon him, was asked about fasting the day of Arafah. He said: "An expiation for two years." And he was asked about fasting the day of Ashura. He said: "An expiation for one year."