What did the Prophet Muhammad (peace be upon him) say about observing fasts for different lengths of time?
 Abu Qatada reported that a person came to the Apostle of Allah (may peace be upon him) and said:                      How do you observe fast? The Messenger of Allah (may peace be upon him) felt annoyed. 1560 When 'Umar (Allah be pleased with him) noticed his annoyance, he said: We are well pleased with Allah as our Lord, with Islam as our Code of Life, and with Muhammad as our Prophet. We seek refuge with Allah from the anger of Allah and that of His Messenger. 'Umar kept on repeating these words till his (the Prophet's) anger calmed down. Then Umar said: Messenger of Allah. what is the position of one who perpetually observes fasts? Thereupon he said: He neither fasted nor broke it, or he said: He did not fast and he did not break it. He said: What about him who observes fast for two days and breaks one day. There- upon he said: Is anyone capable of doing it? He ('Umar) said: What is the position of him who observes fast for a day and breaks on the other day? Thereupon he (the Holy Prophet) said: That is the fast of David (peace be upon him). He ('Umar) said: What about him who observes fast one day and breaks it for two days. Thereupon he (the Messenger of Allah) said: I wish, I were given strength to observe that. Thereafter he said: The observance of three days' fast every Month and that of Ramadan every year is a perpetual fasting. I seek from Allah that fasting on the day of 'Arafa may atone for the sins of the preceding and the coming years. and I seek from Allah that fasting on the day of Ashura may atone for the sins of the preceding year.