It is narrated from Sayyiduna Ibn Amr (may Allah be pleased with him) that once I met the Prophet Muhammad (peace and blessings be upon him), and the Prophet (peace and blessings be upon him) said: "Are you the one about whom I have been informed that you say, 'I will stand (in prayer) every night and fast every day'?" I replied, "Yes, O Messenger of Allah! I have said this." The Prophet (peace and blessings be upon him) said: "Pray (at night) and also sleep; fast and also leave fasting, and fast three days in every month—you will have the reward of fasting for a lifetime." I said, "O Messenger of Allah! I have the strength to do more than this." The Prophet (peace and blessings be upon him) said: "Then fast one day and leave fasting for two days." I said, "O Messenger of Allah! I have the strength to do better than this." The Prophet (peace and blessings be upon him) said: "Then fast one day and leave fasting for one day; this is the most moderate way of fasting, and this is the way of fasting of Sayyiduna Dawood (peace be upon him)." I said, "I have the strength to do even better than this." The Prophet (peace and blessings be upon him) said: "There is no fasting better than this."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 6760