Abu Qatada reported,

The Messenger of Allah ﷺ was asked about his fasting, and the Messenger of Allah ﷺ became angry.Umar said, "We are pleased with Allah as our Lord, with Islam as our religion, and with Muhammad as our Messenger, and our pledge of allegiance stands."He was then asked about fasting two days and breaking fast for one day. He replied, "Who can manage that?"He was asked about fasting one day and breaking fast for two days. He replied, "I wish Allah would give us strength for that."He was asked about fasting one day and breaking fast the next. He replied, "That is the fast of my brother Dawud ."He was asked about fasting on Mondays. He replied, "That is the day I was born, and the day I was sent (or the day revelation came down to me)."He said, "Fasting three days of every month, and from Ramadan to Ramadan, is like fasting the entire year."He was asked about fasting on the Day of Arafah. He replied, "It expiates the past year and the year to come."He was asked about fasting on the Day of Ashura. He replied, "It expiates the past year."

عن أبي قتادة الأنصاري

أن رسول الله ﷺ سئل عن صومه؟ قال: فغضب رسول الله ﷺ. فقال عمر رضي الله عنه: رضينا بالله ربا، وبالإسلام دينا، وبمحمد رسولا، وبيعتنا بيعة. قال: فسئل عن صوم يومين وإفطار يوم؟ قال: ومن يطيق ذلك؟ قال: وسئل عن صوم يوم وإفطار يومين؟ قال: ليت أن الله قوانا لذلك. قال: وسئل عن صوم يوم وإفطار يوم؟ قال: ذاك صوم أخي (داود عليه السلام). قال: وسئل عن صوم الاثنين؟ قال: ذاك يوم ولدت فيه. ويوم بعثت (أو أنزل علي فيه) قال: فقال: صوم ثلاثة من كل شهر، ورمضان إلى رمضان، صوم الدهر. قال: وسئل عن صوم يوم عرفة؟ فقال: يكفر السنة الماضية والباقية. قال: وسئل عن صوم يوم عاشوراء؟ فقال: يكفر السنة الماضية.