كتاب الصيام
13  
The Book of Fasting
(35)
Chapter: Prohibition of Fasting for a lifetime for the one who will be harmed by that or who will neglect other duties, or does not break his fast on the two 'Ids or during the days of At-Tashriq; It is better to fast alternate days
(35)
بَاب النَّهْيِ عَنْ صَوْمِ الدَّهْرِ لِمَنْ تَضَرَّرَ بِهِ أَوْ فَوَّتَ بِهِ حَقًّا أَوْ لَمْ يُفْطِرْ الْعِيدَيْنِ وَالتَّشْرِيقَ وَبَيَانِ تَفْضِيلِ صَوْمِ يَوْمٍ وَإِفْطَارِ يَوْمٍ
Sahih Muslim 1159 n

'Abdullah b. Amr (Allah be pleased with them) reported that the Messenger of Allah (ﷺ) had said to him:

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.
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا غُنْدَرٌ، عَنْ شُعْبَةَ، ح وَحَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، حَدَّثَنَا مُحَمَّدُ بْنُ جَعْفَرٍ، حَدَّثَنَا شُعْبَةُ، عَنْ زِيَادِ بْنِ فَيَّاضٍ، قَالَ سَمِعْتُ أَبَا عِيَاضٍ، عَنْ عَبْدِ، اللَّهِ بْنِ عَمْرٍو - رضى الله عنهما - أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم 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 ‏"‏ ‏.‏
Reference : Sahih Muslim 1159n
In-book reference : Book 13, Hadith 248
USC-MSA web (English) reference : Book 6, Hadith 2598
  (deprecated numbering scheme)