Sunan an-Nasa’i 2397
The Book of Fasting
Fasting ten days of the month, and the Different Wording Reported by the Narrators in the Narration Of 'Abdullah Bin 'Amr About It
It was narrated that 'Abdullah bin 'Amr said: "The Messenger of Allah said: 'I have heard that you stand (in prayer) all night and fast all day.' I said: 'O Messenger of Allah, I only meant good by that.' He said: 'One who fasts every day of his life has not truly fasted. But I will tell you what fasting for a lifetime means: Three days of every month.' I said: 'O Messenger of Allah, I said: 'I am able to do more than that.' He said 'Observe the fast of Dawud, peace be upon him; he used to fast one day and not the next."' 'Ata said: "someone who heard him told me that Ibn 'Umar (said) that the Prophet (ﷺ) said: 'Whoever fasts every day of his life, then he has not fasted."