"The Messenger of Allah took a covenant from me for three: To not sleep except after performing Witr, to fast three days of every month, and to perform the Duha prayer."  

عَهِدَ إِلَىَّ النَّبِيُّ ﷺ ثَلاَثَةً أَنْ لاَ أَنَامَ إِلاَّ عَلَى وِتْرٍ وَصَوْمَ ثَلاَثَةِ أَيَّامٍ مِنْ كُلِّ شَهْرٍ وَأَنْ أُصَلِّيَ الضُّحَى  

the Messenger of Allah said: "O Abu Dharr! When you fast three days out of a month, then fast the thirteenth, fourteenth, and fifteenth."  

قَالَ رَسُولُ اللَّهِ ﷺ يَا أَبَا ذَرٍّ إِذَا صُمْتَ مِنَ الشَّهْرِ ثَلاَثَةَ أَيَّامٍ فَصُمْ ثَلاَثَ عَشْرَةَ وَأَرْبَعَ عَشْرَةَ وَخَمْسَ عَشْرَةَ وَفِي الْبَابِ عَنْ أَبِي قَتَادَةَ وَعَبْدِ اللَّهِ بْنِ عَمْرٍو وَقُرَّةَ بْنِ إِيَاسٍ الْمُزَنِيِّ وَعَبْدِ اللَّهِ بْنِ مَسْعُودٍ وَأَبِي عَقْرَبٍ وَابْنِ عَبَّاسٍ وَعَائِشَةَ وَقَتَادَةَ بْنِ مِلْحَانَ وَعُثْمَانَ بْنِ أَبِي الْعَاصِي وَجَرِيرٍ  

"The Messenger of Allah said: 'Whoever fasts three days in every month, then that is (similar to) fasting every day.' Then Allah Mighty and Sublime is He, attested to that in His Book, by revealing: Whoever brings a good deed, shall have ten times the like thereof. So a day is like ten."  

قَالَ رَسُولُ اللَّهِ ﷺ مَنْ صَامَ مِنْ كُلِّ شَهْرٍ ثَلاَثَةَ أَيَّامٍ فَذَلِكَ صِيَامُ الدَّهْرِ فَأَنْزَلَ اللَّهُ ﷻ تَصْدِيقَ ذَلِكَ فِي كِتَابِهِ (مَنْ جَاءَ بِالْحَسَنَةِ فَلَهُ عَشْرُ أَمْثَالِهَا ) الْيَوْمُ بِعَشَرَةِ أَيَّامٍ  

"I heard Mu'adhah saying to Aishah: 'Did the Messenger of Allah fast three days of every month?' She replied in the affirmative. So she said: 'Which of them would he fast?' She said: 'Which of them he would fast was not noticeable.'"  

قُلْتُ لِعَائِشَةَ أَكَانَ رَسُولُ اللَّهِ ﷺ يَصُومُ ثَلاَثَةَ أَيَّامٍ مِنْ كُلِّ شَهْرٍ قَالَتْ نَعَمْ قُلْتُ مِنْ أَيِّهِ كَانَ يَصُومُ قَالَتْ كَانَ لاَ يُبَالِي مِنْ أَيِّهِ صَامَ