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Narrated `Abdullah bin `Amr:

Allah's Messenger (ﷺ) entered upon me and said, "Have I not been informed that you offer prayer all the night and fast the whole day?" I said, "Yes." He said, "Do not do so; Offer prayer at night and also sleep; Fast for a few days and give up fasting for a few days because your body has a right on you, and your eye has a right on you, and your guest has a right on you, and your wife has a right on you. I hope that you will have a long life, and it is sufficient for you to fast for three days a month as the reward of a good deed, is multiplied ten times, that means, as if you fasted the whole year." I insisted (on fasting more) so I was given a hard instruction. I said, "I can do more than that (fasting)" The Prophet said, "Fast three days every week." But as I insisted (on fasting more) so I was burdened. I said, "I can fast more than that." The Prophet (ﷺ) said, "Fast as Allah's prophet David used to fast." I said, "How was the fasting of the prophet David?" The Prophet (ﷺ) said, "One half of a year (i.e. he used to fast on alternate days). '

حَدَّثَنَا إِسْحَاقُ بْنُ مَنْصُورٍ، حَدَّثَنَا رَوْحُ بْنُ عُبَادَةَ، حَدَّثَنَا حُسَيْنٌ، عَنْ يَحْيَى بْنِ أَبِي كَثِيرٍ، عَنْ أَبِي سَلَمَةَ بْنِ عَبْدِ الرَّحْمَنِ، عَنْ عَبْدِ اللَّهِ بْنِ عَمْرٍو، قَالَ دَخَلَ عَلَىَّ رَسُولُ اللَّهِ صلى الله عليه وسلم فَقَالَ ‏"‏ أَلَمْ أُخْبَرْ أَنَّكَ تَقُومُ اللَّيْلَ وَتَصُومُ النَّهَارَ ‏"‏‏.‏ قُلْتُ بَلَى‏.‏ قَالَ ‏"‏ فَلاَ تَفْعَلْ، قُمْ وَنَمْ، وَصُمْ وَأَفْطِرْ، فَإِنَّ لِجَسَدِكَ عَلَيْكَ حَقًّا، وَإِنَّ لِعَيْنِكَ عَلَيْكَ حَقًّا، وَإِنَّ لِزَوْرِكَ عَلَيْكَ حَقًّا، وَإِنَّ لِزَوْجِكَ عَلَيْكَ حَقًّا، وَإِنَّكَ عَسَى أَنْ يَطُولَ بِكَ عُمُرٌ، وَإِنَّ مِنْ حَسْبِكَ أَنْ تَصُومَ مِنْ كُلِّ شَهْرٍ ثَلاَثَةَ أَيَّامٍ، فَإِنَّ بِكُلِّ حَسَنَةٍ عَشْرَ أَمْثَالِهَا فَذَلِكَ الدَّهْرُ كُلُّهُ ‏"‏‏.‏ قَالَ فَشَدَّدْتُ فَشُدِّدَ عَلَىَّ فَقُلْتُ فَإِنِّي أُطِيقُ غَيْرَ ذَلِكَ‏.‏ قَالَ ‏"‏ فَصُمْ مِنْ كُلِّ جُمُعَةٍ ثَلاَثَةَ أَيَّامٍ ‏"‏‏.‏ قَالَ فَشَدَّدْتُ فَشُدِّدَ عَلَىَّ قُلْتُ أُطِيقُ غَيْرَ ذَلِكَ‏.‏ قَالَ ‏"‏ فَصُمْ صَوْمَ نَبِيِّ اللَّهِ دَاوُدَ ‏"‏‏.‏ قُلْتُ وَمَا صَوْمُ نَبِيِّ اللَّهِ دَاوُدَ قَالَ ‏"‏ نِصْفُ الدَّهْرِ ‏"‏‏.‏
Reference : Sahih al-Bukhari 6134
In-book reference : Book 78, Hadith 161
USC-MSA web (English) reference : Vol. 8, Book 73, Hadith 155
  (deprecated numbering scheme)