It was narrated that 'Abdullah bin 'Amr said:

"The Messenger of Allah said: 'I have heard that you stand (in prayer) all night and fast all day.' I said: 'O Messenger of Allah, I only meant good by that.' He said: 'One who fasts every day of his life has not truly fasted. But I will tell you what fasting for a lifetime means: Three days of every month.' I said: 'O Messenger of Allah, I said: 'I am able to do more than that.' He said 'Observe the fast of Dawud, peace be upon him; he used to fast one day and not the next."' 'Ata said: "someone who heard him told me that Ibn 'Umar (said) that the Prophet ﷺ said: 'Whoever fasts every day of his life, then he has not fasted."

أَخْبَرَنَا مُحَمَّدُ بْنُ عُبَيْدٍ، عَنْ أَسْبَاطٍ، عَنْ مُطَرِّفٍ، عَنْ حَبِيبِ بْنِ أَبِي ثَابِتٍ، عَنْ أَبِي الْعَبَّاسِ، عَنْ عَبْدِ اللَّهِ بْنِ عَمْرٍو، قَالَ قَالَ رَسُولُ اللَّهِ ﷺ " إِنَّهُ بَلَغَنِي أَنَّكَ تَقُومُ اللَّيْلَ وَتَصُومُ النَّهَارَ " . قُلْتُ يَا رَسُولَ اللَّهِ مَا أَرَدْتُ بِذَلِكَ إِلاَّ الْخَيْرَ . قَالَ " لاَ صَامَ مَنْ صَامَ الأَبَدَ وَلَكِنْ أَدُلُّكَ عَلَى صَوْمِ الدَّهْرِ ثَلاَثَةُ أَيَّامٍ مِنَ الشَّهْرِ " . قُلْتُ يَا رَسُولَ اللَّهِ إِنِّي أُطِيقُ أَكْثَرَ مِنْ ذَلِكَ . قَالَ " صُمْ خَمْسَةَ أَيَّامٍ " . قُلْتُ إِنِّي أُطِيقُ أَكْثَرَ مِنْ ذَلِكَ . قَالَ " فَصُمْ عَشْرًا " . فَقُلْتُ إِنِّي أُطِيقُ أَكْثَرَ مِنْ ذَلِكَ . قَالَ " صُمْ صَوْمَ دَاوُدَ عَلَيْهِ السَّلاَمُ كَانَ يَصُومُ يَوْمًا وَيُفْطِرُ يَوْمًا " .