Sunan an-Nasa'i 2403
'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."
أَخْبَرَنَا إِبْرَاهِيمُ بْنُ الْحَسَنِ، قَالَ حَدَّثَنَا حَجَّاجُ بْنُ مُحَمَّدٍ، قَالَ حَدَّثَنِي شُعْبَةُ، عَنْ زِيَادِ بْنِ فَيَّاضٍ، قَالَ سَمِعْتُ أَبَا عِيَاضٍ، قَالَ قَالَ عَبْدُ اللَّهِ بْنُ عَمْرٍو قَالَ لِي رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ صُمْ مِنَ الشَّهْرِ يَوْمًا وَلَكَ أَجْرُ مَا بَقِيَ ‏"‏ ‏.‏ قُلْتُ إِنِّي أُطِيقُ أَكْثَرَ مِنْ ذَلِكَ ‏.‏ قَالَ ‏"‏ فَصُمْ يَوْمَيْنِ وَلَكَ أَجْرُ مَا بَقِيَ ‏"‏ ‏.‏ قُلْتُ إِنِّي أُطِيقُ أَكْثَرَ مِنْ ذَلِكَ ‏.‏ قَالَ ‏"‏ فَصُمْ ثَلاَثَةَ أَيَّامٍ وَلَكَ أَجْرُ مَا بَقِيَ ‏"‏ ‏.‏ قُلْتُ إِنِّي أُطِيقُ أَكْثَرَ مِنْ ذَلِكَ ‏.‏ قَالَ ‏"‏ صُمْ أَرْبَعَةَ أَيَّامٍ وَلَكَ أَجْرُ مَا بَقِيَ ‏"‏ ‏.‏ قُلْتُ إِنِّي أُطِيقُ أَكْثَرَ مِنْ ذَلِكَ ‏.‏ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ أَفْضَلُ الصَّوْمِ صَوْمُ دَاوُدَ كَانَ يَصُومُ يَوْمًا وَيُفْطِرُ يَوْمًا ‏"‏ ‏.‏