The Book of Fasting
13/35. 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
Sahih Muslim
وَحَدَّثَنَا يَحْيَى بْنُ يَحْيَى، أَخْبَرَنَا خَالِدُ بْنُ عَبْدِ اللَّهِ، عَنْ خَالِدٍ، عَنْ أَبِي قِلاَبَةَ، قَالَ أَخْبَرَنِي أَبُو الْمَلِيحِ، قَالَ دَخَلْتُ مَعَ أَبِيكَ عَلَى عَبْدِ اللَّهِ بْنِ عَمْرٍو فَحَدَّثَنَا أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم ذُكِرَ لَهُ صَوْمِي فَدَخَلَ عَلَىَّ فَأَلْقَيْتُ لَهُ وِسَادَةً مِنْ أَدَمٍ حَشْوُهَا لِيفٌ فَجَلَسَ عَلَى الأَرْضِ وَصَارَتِ الْوِسَادَةُ بَيْنِي وَبَيْنَهُ فَقَالَ لِي " أَمَا يَكْفِيكَ مِنْ كُلِّ شَهْرٍ ثَلاَثَةُ أَيَّامٍ " . قُلْتُ يَا رَسُولَ اللَّهِ . قَالَ " خَمْسًا " . قُلْتُ يَا رَسُولَ اللَّهِ . قَالَ " سَبْعًا " . قُلْتُ يَا رَسُولَ اللَّهِ . قَالَ " تِسْعًا " . قُلْتُ يَا رَسُولَ اللَّهِ . قَالَ " أَحَدَ عَشَرَ " . قُلْتُ يَا رَسُولَ اللَّهِ . فَقَالَ النَّبِيُّ صلى الله عليه وسلم " لاَ صَوْمَ فَوْقَ صَوْمِ دَاوُدَ شَطْرُ الدَّهْرِ صِيَامُ يَوْمٍ وَإِفْطَارُ يَوْمٍ " .
Abu Qatada reported that Abu al Malih informed me: I went along with your father to 'Abdullah b. Amr, and he narrated to us that the Messenger of Allah (ﷺ) was informed about my fasting and he came to me, and I placed a leather cushion filled with fibre of date-palms for him. He sat down upon the ground and there was that cushion between me and him, and he said to me: Does three days' fasting in a month not suffice you? I said: Messenger of Allah, (I am capable of observing more fasts). He said: (Would) five (not suffice for you)? I said: Messenger of Allah, (I am capable of observing more fasts) He said: (Would) seven (fasts) not suffice you? I said: Messenger of Allah, (I am capable of observing more fasts). He (the Holy Prophet) then said: (Would) nine (fasts not suffice you)? I said: Messenger of Allah, (I am capable of observing more fasts). He said: (Would) eleven (fasts not suffice you)? I said: Messenger of Allah, (I am capable of observing more fasts than these). Thereupon the Messenger of Allah (ﷺ) said: There is no fasting (better than) the fasting of David which comprises half of the age, fasting a day and not fasting a day.
Reference : Sahih Muslim 1159mIn-book reference : Book 13, Hadith 247USC-MSA web (English) reference : Book 6, Hadith 2597