Sunan an-Nasā'ī 2409✓ 5Sahih
أَخْبَرَنَا عَلِيُّ بْنُ الْحَسَنِ اللاَّنِيُّ، بِالْكُوفَةِ عَنْ عَبْدِ الرَّحِيمِ، - وَهُوَ ابْنُ سُلَيْمَانَ - عَنْ عَاصِمٍ الأَحْوَلِ، عَنْ أَبِي عُثْمَانَ، عَنْ أَبِي ذَرٍّ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " مَنْ صَامَ ثَلاَثَةَ أَيَّامٍ مِنَ الشَّهْرِ فَقَدْ صَامَ الدَّهْرَ كُلَّهُ " . ثُمَّ قَالَ " صَدَقَ اللَّهُ فِي كِتَابِهِ { مَنْ جَاءَ بِالْحَسَنَةِ فَلَهُ عَشْرُ أَمْثَالِهَا } " .
It was narrated that Abu Dharr said:"The Messenger of Allah said: 'Whoever fasts for three days of each month, he has fasted for a whole lifetime.' Then he said: Allah has spoken the truth in His book: Whoever brings a good deed shall have ten times the like thereof to his credit