Sunan an-Nasa'i 2387

It was narrated that Abu Qatadah said:"Umar said: 'O Messenger of Allah, what about a person who fasted for an entire lifetime?' He said: 'He neither fasted nor broke his fast for one day?' He said: 'Can anyone do that?' He said: 'What about a person who fasted for one day?' He said: 'That is the fast of Dawud, peace be upon him.' He said: 'What about a person who fasted for one day, and broke his fast for two days?' he said: 'I wish that I could do that.' Then he said: 'Three days of each month, and from Ramadan to Ramadan, this is fasting for an entire lifetime."' '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