Sunan an-Nasa'i 2387
It was narrated that Abu Qatadah said:
"Umar said: 'O Messenger of Allah, what about a person who fasted for an entire lifetime?' He said: 'He neither fasted nor broke his fast for one day?' He said: 'Can anyone do that?' He said: 'What about a person who fasted for one day?' He said: 'That is the fast of Dawud, peace be upon him.' He said: 'What about a person who fasted for one day, and broke his fast for two days?' he said: 'I wish that I could do that.' Then he said: 'Three days of each month, and from Ramadan to Ramadan, this is fasting for an entire lifetime."' 'Ata said: "someone who heard him told me that Ibn 'Umar (said) that the Prophet said: 'Whoever fasts every day of his life, then he has not fasted."
أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا حَمَّادٌ، عَنْ غَيْلاَنَ بْنِ جَرِيرٍ، عَنْ عَبْدِ اللَّهِ بْنِ مَعْبَدٍ الزِّمَّانِيِّ، عَنْ أَبِي قَتَادَةَ، قَالَ قَالَ عُمَرُ يَا رَسُولَ اللَّهِ كَيْفَ بِمَنْ يَصُومُ الدَّهْرَ كُلَّهُ قَالَ ‏"‏ لاَ صَامَ وَلاَ أَفْطَرَ أَوْ لَمْ يَصُمْ وَلَمْ يُفْطِرْ ‏"‏ ‏.‏ قَالَ يَا رَسُولَ اللَّهِ كَيْفَ بِمَنْ يَصُومُ يَوْمَيْنِ وَيُفْطِرُ يَوْمًا قَالَ ‏"‏ أَوَيُطِيقُ ذَلِكَ أَحَدٌ ‏"‏ ‏.‏ قَالَ فَكَيْفَ بِمَنْ يَصُومُ يَوْمًا وَيُفْطِرُ يَوْمًا قَالَ ‏"‏ ذَلِكَ صَوْمُ دَاوُدَ عَلَيْهِ السَّلاَمُ ‏"‏ ‏.‏ قَالَ فَكَيْفَ بِمَنْ يَصُومُ يَوْمًا وَيُفْطِرُ يَوْمَيْنِ قَالَ ‏"‏ وَدِدْتُ أَنِّي أُطِيقُ ذَلِكَ ‏"‏ ‏.‏ قَالَ ثُمَّ قَالَ ‏"‏ ثَلاَثٌ مِنْ كُلِّ شَهْرٍ وَرَمَضَانُ إِلَى رَمَضَانَ هَذَا صِيَامُ الدَّهْرِ كُلِّهِ ‏"‏ ‏.‏