Narrated 'Abd Allah b. 'Amr b. al-'As:
The Messenger of Allah ﷺ met me and said: Have I not been informed that you told: I shall stand at prayer all the night, and I shall fast during the day ? He said: I think so. Yes, Messenger of Allah, I have said this. He said: Get up and pray at night and sleep ; fast and break your fast ; fast three days every month: that is equivalent to keeping perpetual fast. I said: Messenger of Allah, I have more power than that. He said: Then fast one day and break your fast one day. That is the most moderate fast ; that is the fast of Dawud (David). He said: I have more power than that. The Messenger of Allah ﷺ said: There is no fast more excellent that it.

حَدَّثَنَا الْحَسَنُ بْنُ عَلِيٍّ، حَدَّثَنَا عَبْدُ الرَّزَّاقِ، حَدَّثَنَا مَعْمَرٌ، عَنِ الزُّهْرِيِّ، عَنِ ابْنِ الْمُسَيَّبِ، وَأَبِي، سَلَمَةَ عَنْ عَبْدِ اللَّهِ بْنِ عَمْرِو بْنِ الْعَاصِ، قَالَ لَقِيَنِي رَسُولُ اللَّهِ ﷺ فَقَالَ " أَلَمْ أُحَدَّثْ أَنَّكَ تَقُولُ لأَقُومَنَّ اللَّيْلَ وَلأَصُومَنَّ النَّهَارَ " . قَالَ - أَحْسِبُهُ قَالَ - نَعَمْ يَا رَسُولَ اللَّهِ قَدْ قُلْتُ ذَاكَ . قَالَ " قُمْ وَنَمْ وَصُمْ وَأَفْطِرْ وَصُمْ مِنْ كُلِّ شَهْرٍ ثَلاَثَةَ أَيَّامٍ وَذَاكَ مِثْلُ صِيَامِ الدَّهْرِ " . قَالَ قُلْتُ يَا رَسُولَ اللَّهِ إِنِّي أُطِيقُ أَفْضَلَ مِنْ ذَلِكَ . قَالَ " فَصُمْ يَوْمًا وَأَفْطِرْ يَوْمَيْنِ " . قَالَ فَقُلْتُ إِنِّي أُطِيقُ أَفْضَلَ مِنْ ذَلِكَ قَالَ " فَصُمْ يَوْمًا وَأَفْطِرْ يَوْمًا وَهُوَ أَعْدَلُ الصِّيَامِ وَهُوَ صِيَامُ دَاوُدَ " . قُلْتُ إِنِّي أُطِيقُ أَفْضَلَ مِنْ ذَلِكَ فَقَالَ رَسُولُ اللَّهِ ﷺ " لاَ أَفْضَلَ مِنْ ذَلِكَ " .