2403. Sunan an-Nasa’i Chapter: The Obligation of Fasting; Book 22, Hadith 314

‘Abdullah bin Amr said: “The Messenger of Allah said to me: ‘Fast one day of the month and you will have the reward of what is left.’ I said: ‘I am able to do more than that.’ He said: ‘Fast two days and you will have the reward of what is left.’ I said: ‘I am able to do more than that.’ He said: ‘Fast three days and you will have the reward of what is left.’ I said: ‘I am able to do more than that.’ He said: ‘Fast four days and you will have the reward of what is left.’ I said: ‘ I am able to do more than that.’ The messenger of Allah said: ‘The best fasting is the fast of Dawud; he used to fast one day and not the next.”‘
2406. It was narrated that Abu Dharr said: “My beloved Prophet advised me to do three
things which I will never give up, if Allah wills. He advised me to pray Duha to pray Witr before sleeping, and to fast three days of each month.”