Mu'adha al-‘Adawiya said she asked ‘A'isha whether God’s messenger used to fast three days every month, and she replied that he did. She asked which days in the month he fasted, and she replied that he did not care which days of the month he fasted.
He said that when God’s messenger was asked about fasting on Monday he said, “On it I was born and on it the revelation was first sent down to me.”*
*The meaning seems to be that as this day is so distinguished, one could not choose a better one.
Abu Ayyub al-Ansari told that God’s messenger said, “If anyone fasts during Ramadan, then follows it with six days in Shawwal, it will be like a perpetual fast.”
Abu Qatada said that when a man came to the Prophet and asked him how he fasted, he was angry at what he said, and when 'Umar observed his anger he said, “We are satisfied with God as Lord, with Islam as religion and with Muhammad as Prophet. We seek refuge in God from the anger of God and His messenger.” ‘Umar kept on repeating these words till his anger calmed down, then asked, “Messenger of God, what is the position of one who observes a perpetual fast?” He replied, “May he not fast or break his fast!” or he said, “He has neither fasted nor broken his fast.” He asked, “What is the position of one who fasts two days out of every three?” and received the reply, “Is anyone able to do that?” He asked what was the position of one who fasted every second day and was told that was the fast David observed. He asked what was the position of one who fasted one day out of every three, and God’s messenger replied, “I wish I were given power to observe that.” Thereafter he said, “The observance of three days’ fast every month and of Ramadan every year is a perpetual fast. I seek from God that fasting on the day of ‘Arafa may atone for the sins of the preceding and the coming year, and I seek from God that fasting on the day of ‘Ashura’ may atone for the sins of the preceding year.”
