Sunan an-Nasa'i 2394
It was narrated from 'Abdullah bin 'Amr that the Messenger of Allah (ﷺ) said to him:
"Fast one day, and you will have the reward of what is left." He said: "I am able to do more than that." He (ﷺ) said: "Fast two days, and you will have the reward of what is left." He said: "I am able to do more than that." He (ﷺ) said: "Fast three days and you will have the reward of what is left." He said: "I am able to do more than that." He (ﷺ) said: "Fast four days and you will have the reward of what is left." He said: "I am able to do more than that." He (ﷺ) said: "Observe the best kind of fasting before Allah, the fast of Dawud, peace be upon him; he used to fast one day and break his 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 الْمُثَنَّى، قَالَ حَدَّثَنَا مُحَمَّدٌ، قَالَ حَدَّثَنَا شُعْبَةُ، عَنْ زِيَادِ بْنِ فَيَّاضٍ، سَمِعْتُ أَبَا عِيَاضٍ، يُحَدِّثُ عَنْ عَبْدِ اللَّهِ بْنِ عَمْرٍو، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ لَهُ ‏"‏ صُمْ يَوْمًا وَلَكَ أَجْرُ مَا بَقِيَ ‏"‏ ‏.‏ قَالَ إِنِّي أُطِيقُ أَكْثَرَ مِنَ ذَلِكَ ‏.‏ قَالَ ‏"‏ صُمْ يَوْمَيْنِ وَلَكَ أَجْرُ مَا بَقِيَ ‏"‏ ‏.‏ قَالَ إِنِّي أُطِيقُ أَكْثَرَ مِنْ ذَلِكَ ‏.‏ قَالَ ‏"‏ صُمْ ثَلاَثَةَ أَيَّامٍ وَلَكَ أَجْرُ مَا بَقِيَ ‏"‏ ‏.‏ قَالَ إِنِّي أُطِيقُ أَكْثَرَ مِنْ ذَلِكَ ‏.‏ قَالَ ‏"‏ صُمْ أَرْبَعَةَ أَيَّامٍ وَلَكَ أَجْرُ مَا بَقِيَ ‏"‏ ‏.‏ قَالَ إِنِّي أُطِيقُ أَكْثَرَ مِنْ ذَلِكَ ‏.‏ قَالَ ‏"‏ صُمْ أَفْضَلَ الصِّيَامِ عِنْدَ اللَّهِ صَوْمَ دَاوُدَ عَلَيْهِ السَّلاَمُ كَانَ يَصُومُ يَوْمًا وَيُفْطِرُ يَوْمًا ‏"‏ ‏.‏