“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  

[Machine] The Messenger of Allah ﷺ said, "Whoever fasts three days of the month, let him fast the entire month." Then he said, "Allah and His Messenger spoke the truth in His Book, 'Whoever comes with a good deed will have ten times its like.'" [Al-Anam: 160]  

قَالَ رَسُولُ اللهِ ﷺ «مَنْ صَامَ ثَلَاثَةَ أَيَّامٍ مِنَ الشَّهْرِ فَلْيَصُمِ الدَّهْرَ كُلَّهُ» ثُمَّ قَالَ صَدَقَ اللهُ وَرَسُولُهُ فِي كِتَابِهِ {مَنْ جَاءَ بِالْحَسَنَةِ فَلَهُ عَشْرُ أَمْثَالِهَا} [الأنعام 160]  

" عَنْ عَلىٍّ عَنِ النبِيّ ﷺ قَالَ: صِيامُ ثَلاَثَةِ أَيَّامٍ مِنْ كلِّ شهْرٍ صيَامُ الدَّهْرِ، كلُّ يَوْمٍ عَشْرَةُ أَيَّامٍ، مَنْ جاءَ بِالْحَسَنَةِ فَلَهُ عَشْرُ أَمْثالِها ".