It was narrated that 'Abdullah bin 'Amr said:

"The Messenger of Allah said to me: 'O 'Abdullah bin 'Amr, you fast all the time and you do stand (in prayer) at night, but if you do that your eyes will become sunken and you will become exhausted. There is no fast for one who fasts every day of his life. Fasting a lifetime means fasting three days each month, (that is) fasting all of a lifetime.' I said: 'I am able to do more than that.' He said: 'Observe the fast of Dāwūd; he used to fast one day and not the next, and he did not flee if he met (the enemy in battle)'"

أَخْبَرَنَا مُحَمَّدُ بْنُ عَبْدِ الأَعْلَى، قَالَ حَدَّثَنَا خَالِدٌ، حَدَّثَنَا شُعْبَةُ، قَالَ أَخْبَرَنِي حَبِيبُ بْنُ أَبِي ثَابِتٍ، قَالَ سَمِعْتُ أَبَا الْعَبَّاسِ، - هُوَ الشَّاعِرُ - يُحَدِّثُ عَنْ عَبْدِ اللَّهِ بْنِ عَمْرٍو، قَالَ قَالَ لِي رَسُولُ اللَّهِ ﷺ " يَا عَبْدَ اللَّهِ بْنَ عَمْرٍو إِنَّكَ تَصُومُ الدَّهْرَ وَتَقُومُ اللَّيْلَ وَإِنَّكَ إِذَا فَعَلْتَ ذَلِكَ هَجَمَتِ الْعَيْنُ وَنَفِهَتْ لَهُ النَّفْسُ لاَ صَامَ مَنْ صَامَ الأَبَدَ صَوْمُ الدَّهْرِ ثَلاَثَةُ أَيَّامٍ مِنَ الشَّهْرِ صَوْمُ الدَّهْرِ كُلِّهِ " . قُلْتُ إِنِّي أُطِيقُ أَكْثَرَ مِنْ ذَلِكَ . قَالَ " صُمْ صَوْمَ دَاوُدَ كَانَ يَصُومُ يَوْمًا وَيُفْطِرُ يَوْمًا وَلاَ يَفِرُّ إِذَا لاَقَى " .