It was narrated from 'Abdullah bin 'Amr that the Messenger of Allah (ﷺ) said to him:
"Fast one day, and you will have the reward of what is left." He said: "I am able to do more than that." He (ﷺ) said: "Fast two days, and you will have the reward of what is left." He said: "I am able to do more than that." He (ﷺ) said: "Fast three days and you will have the reward of what is left." He said: "I am able to do more than that." He (ﷺ) said: "Fast four days and you will have the reward of what is left." He said: "I am able to do more than that." He (ﷺ) said: "Observe the best kind of fasting before Allah, the fast of Dawud, peace be upon him; he used to fast one day and break his fast for one day." 'Ata said: "Someone who heard him told me that Ibn 'Umar (said) that the Prophet said: 'Whoever fasts every day of his life, then he has not fasted.'"
Reference: Sunan an-Nasa'i 2394 In-book : Book 22, Hadith 305 English translation : Vol. 3, Book 22, Hadith 2396
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