Sunan An-Nasa'i

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