Sunan an-Nasa'i 2409
سنن النسائي#2409Sahihأَخْبَرَنَا 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
Scholarly Authentication
The label above is the grade most of these scholars gave — one dissenting opinion no longer speaks for all of them. If any scholar calls a narration fabricated, that verdict is shown instead, whatever the others say. Each scholar's own wording:
In-book reference: 22:320