Narrated Abu Qatadah (RA): A man came to the Prophet (peace be upon him) and said: How do you fast, Messenger of Allah (peace be upon him)? The Messenger of Allah (peace be upon him) became angry at what he said. When Umar observed this (his anger), he said: We are satisfied with Allah as Lord, with Islam as religion, and with Muhammad as Prophet (peace be upon him). We seek refuge in Allah from the anger of Allah, and from the anger of His Apostle. Umar continued to repeat these words till his anger cooled down. He then asked: Messenger of Allah, what is the position of one who observes a perpetual fast? He replied: May he not fast or break his fast. Musaddad said in his version: He has neither fasted nor broken his fast. The narrator, Ghaylan, doubted the actual wordings. He asked: What is the position of one who fasts two days and does not fast one day? He said: Is anyone able to do that? He asked: What is the position of one who fasts every second day (ie fasts one day and does not fasts the next day)? He (the Prophet) said: This is the fast that David observed. He asked: Messenger of Allah, what is the position of one who fasts one day and breaks it for two days? He replied: I wish I were given the power to observe that. Then the Messenger of Allah (peace be upon him) said: The observance of three days’ fast every month and of one Ramadan to the other (ie the fast of Ramadan every year) is (equivalent to) a perpetual fast. I seek from Allah that fasting on the day of Arafah may atone for the sins of the preceding and the coming year, and I seek from Allah that fasting on the day of Ashura ‘may atone for the sins of the preceding year.
Sunan Abi Dawud 2425, Book 14, Hadith 113