Musnad Ahmad 1007:105

'Affan narrated to us, Mahdi ibn Maymun narrated to us, Ghaylan ibn Jarir narrated to us, from 'Abdullah ibn Mu'bad al-Zamani, from Abu Qatadah, from the Prophet (peace and blessings of Allah be upon him), that a man said to him: "What do you say about fasting the day of 'Arafah?" He said: "I anticipate from Allah that it will expiate the year that has passed and the year that is to come." He said: "O Messenger of Allah, what do you say about fasting the day of 'Ashura'?" He said: "I anticipate from Allah that it will expiate the year."