Musnad Ahmad 1007:134

Waki' narrated to us, Mahdi ibn Maymun narrated to us, from Ghaylan ibn Jarir, from Abdullah ibn Ma'bad, from Abu Qatadah, that a man asked the Prophet, peace and blessings of Allah be upon him, about fasting the day of Arafah. He said: "I anticipate from Allah that it is an expiation for two years, a past one and a future one." He said: "O Messenger of Allah, what about a man who fasts the entire time?" He said: "He has neither fasted nor broken his fast," or "He did not fast and he did not break his fast." He said: "O Messenger of Allah, what about a man who fasts one day and breaks his fast for one day?" He said: "That is the fasting of my brother Dawud, peace be upon him." He said: "O Messenger of Allah, what about a man who fasts one day and breaks his fast for two days?" He said: "I wish I were enabled to do that." He said: "What about a man who fasts two days and breaks his fast for one day?" He said: "And who can endure that?" He said: And he was asked about fasting the day of Ashura. He said: "I anticipate from Allah that it is an expiation for a year."