The Messenger of Allah ﷺ used to enjoin fasting the bright days – the thirteenth, fourteenth and fifteenth (when the moon is full). He said: “It is like fasting for a lifetime.” Another chain from 'Abdul-Malik bin Qatadah bin Malhan Al-Qaisi, from his father, from the Prophet ﷺ with similar wording. Ibn Majah said: Shu'bah erred (in the name of one of the narrators)and Hammam was correct.  

عَنْ رَسُولِ اللَّهِ ﷺ أَنَّهُ كَانَ يَأْمُرُ بِصِيَامِ الْبِيضِ ثَلاَثَ عَشْرَةَ وَأَرْبَعَ عَشْرَةَ وَخَمْسَ عَشْرَةَ وَيَقُولُ هُوَ كَصَوْمِ الدَّهْرِ أَوْ كَهَيْئَةِ صَوْمِ الدَّهْرِ حَدَّثَنَا إِسْحَاقُ بْنُ مَنْصُورٍ أَنْبَأَنَا حَبَّانُ بْنُ هِلاَلٍ حَدَّثَنَا هَمَّامٌ عَنْ أَنَسِ بْنِ سِيرِينَ حَدَّثَنِي عَبْدُ الْمَلِكِ بْنُ قَتَادَةَ بْنِ مِلْحَانَ الْقَيْسِيُّ عَنْ أَبِيهِ عَنِ النَّبِيِّ ﷺ نَحْوَهُ قَالَ ابْنُ مَاجَهْ أَخْطَأَ شُعْبَةُ وَأَصَابَ هَمَّامٌ  

“Whoever fasts three days in every month, that is fasting for a lifetime.” Then, in testimony of that, Allah revealed: “Whoever brings a good deed shall have ten time the like thereof to his credit.” [6:160] So one day is equivalent to ten (in reward).  

قَالَ رَسُولُ اللَّهِ ﷺ مَنْ صَامَ ثَلاَثَةَ أَيَّامٍ مِنْ كُلِّ شَهْرٍ فَذَلِكَ صَوْمُ الدَّهْرِ فَأَنْزَلَ اللَّهُ ﷻ تَصْدِيقَ ذَلِكَ فِي كِتَابِهِ {مَنْ جَاءَ بِالْحَسَنَةِ فَلَهُ عَشْرُ أَمْثَالِهَا} فَالْيَوْمُ بِعَشْرَةِ أَيَّامٍ  

“The Messenger of Allah ﷺ used to fast three days of each month.” I said: “Which were they?” She said: “He did not care which days they were.”  

أَنَّهَا قَالَتْ كَانَ رَسُولُ اللَّهِ ﷺ يَصُومُ ثَلاَثَةَ أَيَّامٍ مِنْ كُلِّ شَهْرٍ قُلْتُ مِنْ أَيِّهِ؟ قَالَتْ لَمْ يَكُنْ يُبَالِي مِنْ أَيِّهِ كَانَ