Sahih Bukhari Volume 3, Book 30, Hadith 1978

حَدَّثَنَا عَمْرُو بْنُ عَلِيٍّ، أَخْبَرَنَا أَبُو عَاصِمٍ، عَنِ ابْنِ جُرَيْجٍ، سَمِعْتُ عَطَاءً، أَنَّ أَبَا الْعَبَّاسِ الشَّاعِرَ، أَخْبَرَهُ أَنَّهُ، سَمِعَ عَبْدَ اللَّهِ بْنَ عَمْرٍو ـ رضى الله عنهما ـ بَلَغَ النَّبِيَّ صلى الله عليه وسلم أَنِّي أَسْرُدُ الصَّوْمَ وَأُصَلِّي اللَّيْلَ، فَإِمَّا أَرْسَلَ إِلَىَّ، وَإِمَّا لَقِيتُهُ، فَقَالَ ‏"‏ أَلَمْ أُخْبَرْ أَنَّكَ تَصُومُ وَلاَ تُفْطِرُ، وَتُصَلِّي وَلاَ تَنَامُ، فَصُمْ وَأَفْطِرْ، وَقُمْ وَنَمْ، فَإِنَّ لِعَيْنِكَ عَلَيْكَ حَظًّا، وَإِنَّ لِنَفْسِكَ وَأَهْلِكَ عَلَيْكَ حَظًّا ‏"‏‏.‏ قَالَ إِنِّي لأَقْوَى لِذَلِكَ‏.‏ قَالَ ‏"‏ فَصُمْ صِيَامَ دَاوُدَ ـ عَلَيْهِ السَّلاَمُ ـ ‏"‏‏.‏ قَالَ وَكَيْفَ قَالَ ‏"‏ كَانَ يَصُومُ يَوْمًا وَيُفْطِرُ يَوْمًا، وَلاَ يَفِرُّ إِذَا لاَقَى ‏"‏‏.‏ قَالَ مَنْ لِي بِهَذِهِ يَا نَبِيَّ اللَّهِ قَالَ عَطَاءٌ لاَ أَدْرِي كَيْفَ ذَكَرَ صِيَامَ الأَبَدِ، قَالَ النَّبِيُّ صلى الله عليه وسلم ‏"‏ لاَ صَامَ مَنْ صَامَ الأَبَدَ ‏"‏‏.‏ مَرَّتَيْنِ‏

Narrated By 'Abdullah bin 'Amr : The news of my daily fasting and praying every night throughout the night reached the Prophet. So he sent for me or I met him, and he said, "I have been informed that you fast everyday and pray every night (all the night). Fast (for some days) and give up fasting (for some days); pray and sleep, for your eyes have a right on you, and your body and your family (i.e. wife) have a right on you." I replied, "I have more power than that (fasting)." The Prophet said, "Then fast like the fasts of (the Prophet) David". I said, "How?" He replied, "He used to fast on alternate days, and he used not to flee on meeting the enemy." I said, "From where can I get that chance?" ('Ata' said, "I do not know how the expression of fasting daily throughout the life occurred.") So, the Prophet said, twice, "Whoever fasts daily throughout his life is just as the one who does not fast at all."