Sunan an-Nasa'i 2389
It was narrated that Muhajid said:
"Abdullah bin 'Amr said to me: My father got me married to a woman from a noble family, and he used to come to her and ask her about her husband. She said: What a wonderful man he is! He never comes to my bed. And he has never approached me since he married me. He mentioned that to the Prophet and he said: Bring him to me. So he brought him with him and (the Prophet) said: How do you fast? I said: "Every day." He said: "Fast three days of every month." I said: "I am able to do better than that." He said: "Fast for two days, and break your fast for one day." He said; "I am able to do better than that". He said: "Observe the best of fasts, the fast of Dawud, peace be upon him: Fasting for one day and breaking the fast for one day." '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 مُحَمَّدُ بْنُ مَعْمَرٍ، قَالَ حَدَّثَنَا يَحْيَى بْنُ حَمَّادٍ، قَالَ حَدَّثَنَا أَبُو عَوَانَةَ، عَنْ مُغِيرَةَ، عَنْ مُجَاهِدٍ، قَالَ قَالَ لِي عَبْدُ اللَّهِ بْنُ عَمْرٍو أَنْكَحَنِي أَبِي امْرَأَةً ذَاتَ حَسَبٍ فَكَانَ يَأْتِيهَا فَيَسْأَلُهَا عَنْ بَعْلِهَا، فَقَالَتْ نِعْمَ الرَّجُلُ مِنْ رَجُلٍ لَمْ يَطَأْ لَنَا فِرَاشًا وَلَمْ يُفَتِّشْ لَنَا كَنَفًا مُنْذُ أَتَيْنَاهُ ‏.‏ فَذَكَرَ ذَلِكَ لِلنَّبِيِّ صلى الله عليه وسلم فَقَالَ ‏"‏ ائْتِنِي بِهِ ‏"‏ ‏.‏ فَأَتَيْتُهُ مَعَهُ فَقَالَ ‏"‏ كَيْفَ تَصُومُ ‏"‏ ‏.‏ قُلْتُ كُلَّ يَوْمٍ ‏.‏ قَالَ ‏"‏ صُمْ مِنْ كُلِّ جُمُعَةٍ ثَلاَثَةَ أَيَّامٍ ‏"‏ ‏.‏ قُلْتُ إِنِّي أُطِيقُ أَفْضَلَ مِنْ ذَلِكَ ‏.‏ قَالَ ‏"‏ صُمْ يَوْمَيْنِ وَأَفْطِرْ يَوْمًا ‏"‏ ‏.‏ قَالَ إِنِّي أُطِيقُ أَفْضَلَ مِنْ ذَلِكَ ‏.‏ قَالَ ‏"‏ صُمْ أَفْضَلَ الصِّيَامِ صِيَامَ دَاوُدَ عَلَيْهِ السَّلاَمُ صَوْمُ يَوْمٍ وَفِطْرُ يَوْمٍ ‏"‏ ‏.‏