"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." (Using translation from Nasāʾī 2403)  

لِي رَسُولُ اللهِ ﷺ «صُمْ مِنَ الشَّهْرِ يَوْمًا وَلَكَ أَجْرُ مَا بَقِيَ» فَقُلْتُ إِنِّي أُطِيقُ أَكْثَرَ مِنْ ذَلِكَ قَالَ «فَصُمْ يَوْمَيْنِ وَلَكَ أَجْرُ مَا بَقِيَ» فَقُلْتُ إِنِّي أُطِيقُ أَكْثَرَ مِنْ ذَلِكَ قَالَ «صُمْ ثَلَاثَةَ أَيَّامٍ وَلَكَ أَجْرُ مَا بَقِيَ» فَقُلْتُ إِنِّي أُطِيقُ أَكْثَرَ مِنْ ذَلِكَ قَالَ «صُمْ أَرْبَعَةَ أَيَّامٍ وَلَكَ أَجْرُ مَا بَقِيَ» قُلْتُ إِنِّي أُطِيقُ أَكْثَرَ مِنْ ذَلِكَ فَقَالَ رَسُولُ اللهِ ﷺ بَعْدَمَا قَالَ «أَرْبَعَةَ أَيَّامٍ» «أَفْضَلُ الصَّوْمِ صَوْمُ دَاوُدَ كَانَ يَصُومُ يَوْمًا وَيُفْطِرُ يَوْمًا»