Abu Qatada al-Ansari (Allah be pleased with him) reported that the Messenger of Allah (ﷺ) was asked about his fasting. The Messenger of Allah (ﷺ) felt annoyed. Thereupon 'Umar (Allah be pleased with him) said:
We are pleased with Allah as the Lord, with Islam as our Code of Life, with Muhammad as the Messenger and with our pledge (to you for willing and cheerful submission) as a (sacred) commitment. He was then asked about perpetual fasting, whereupon he said: He neither fasted nor did he break it, or he did not fast and he did not break it. He was then asked about fasting for two days and breaking one day. He (the Holy Prophet) said: And who has strength enough to do it? He was asked about fasting for a day and breaking for two days, whereupon he said: May Allah bestow upon us strength to do it. He was then asked about fasting for a day and breaking on the other, whereupon he said: That is the fasting of my brother David (peace be upon him). He was then asked about fasting on Monday, whereupon he said: It was the day on which I was born. on which I was commissioned with prophethood or revelation was sent to me, (and he further) said: Three days' fasting every month and of the whole of Ramadan every year is a perpetual fast. He was asked about fasting on the day of 'Arafa (9th of Dhu'I-Hijja), whereupon he said: It expiates the sins of the preceding year and the coming year. He was asked about fasting on the day of 'Ashura (10th of Muharram), whereupon be said: It expiates the sins of the preceding year. (Imam Muslim said that in this hadith there is a) narration of Imam Shu'ba that he was asked about fasting on Monday and Thursday, but we (Imam Muslim) did not mention Thursday for we found it as an error (in reporting).