Musnad Ahmad 1007:21

Yahya ibn Sa'id narrated to us, Shu'bah narrated to us, Ghaylan ibn Jarir narrated to us, from 'Abdullah ibn Ma'bad al-Zamani, from Abu Qatadah, who said: Shu'bah said: I said to Ghaylan: "The Ansari?" He signaled with his head, meaning yes. A man asked the Prophet, peace and blessings of Allah be upon him, about his fasting, and he became angry. 'Umar said: "We are pleased with Allah as Lord, and with Islam as a religion." He said: And I do not know but that he said: "And with Muhammad as a Messenger, and with our pledge of allegiance as a pledge." He said: Then 'Umar or another man stood up and said: "O Messenger of Allah, a man who fasts forever?" He said: "He has neither fasted nor broken his fast," or "He did not fast and he did not break his fast." He said: "Fasting two days and breaking the fast for one day?" He said: "And who can endure that?" He said: "Breaking the fast for two days and fasting one day?" He said: "I wish Allah, the Almighty and Majestic, would give us the strength for that." He said: "Fasting one day and breaking the fast for one day?" He said: "That is the fast of my brother Dawud." He said: "Fasting on Monday and Thursday?" He said: "That is the day I was born, and the day the revelation was sent down to me." He said: "Fasting three days of every month, and Ramadan to Ramadan, is the fasting of a lifetime and the breaking of its fast." He said: "Fasting the day of 'Arafah?" He said: "It expiates the past year and the remaining year." He said: "Fasting the day of 'Ashura'?" He said: "It expiates the past year."