Observe fast for a day and there would be reward for you for the rest (of the days). He said: I am capable of doing more than this. He then said: Observe fast for two days, and there would be reward for you for the rest (of the days). He said: I am capable of doing more than this. He (the Holy Prophet) said: Observe fast for three days and there would be reward for you for the rest of the days. He said: I am capable of doing more than this, whereupon he said: Observe fast for four days and there would be reward for you for the rest of the days. He said: I am capable of doing more than this. Thereupon he said: Then observe fast (which is the) best in the eye of Allah, the fast of David (peace be upon him) ; he used to observe fast one day and break on the other day.
أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ لَهُ صُمْ يَوْمًا وَلَكَ أَجْرُ مَا بَقِيَ قَالَ إِنِّي أُطِيقُ أَكْثَرَ مِنْ 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