" I heard the Messenger of Allah say: 'The month of patience ( Ramadan) and three days of each month is fasting for a lifetime."' (Sahih).  

سَمِعْتُ رَسُولَ اللَّهِ ﷺ يَقُولُ شَهْرُ الصَّبْرِ وَثَلاَثَةُ أَيَّامٍ مِنْ كُلِّ شَهْرٍ صَوْمُ الدَّهْرِ  

"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.  

قَالَ رَسُولُ اللَّهِ ﷺ مَنْ صَامَ ثَلاَثَةَ أَيَّامٍ مِنَ الشَّهْرِ فَقَدْ صَامَ الدَّهْرَ كُلَّهُ ثُمَّ قَالَ صَدَقَ اللَّهُ فِي كِتَابِهِ { مَنْ جَاءَ بِالْحَسَنَةِ فَلَهُ عَشْرُ أَمْثَالِهَا }  

"I heard the Messenger of Allah say: "Whoever fasts three days of each month has fasted the month in full' or 'will have (the reward of) fasting the month."' 'Asim was in doubt.  

سَمِعْتُ رَسُولَ اللَّهِ ﷺ يَقُولُ مَنْ صَامَ ثَلاَثَةَ أَيَّامٍ مِنْ كُلِّ شَهْرٍ فَقَدْ تَمَّ صَوْمُ الشَّهْرِ أَوْ فَلَهُ صَوْمُ الشَّهْرِ شَكَّ عَاصِمٌ  

"I heard the Messenger of Allah say: 'It is a good fast to fast three days of each month".  

سَمِعْتُ رَسُولَ اللَّهِ ﷺ يَقُولُ صِيَامٌ حَسَنٌ ثَلاَثَةُ أَيَّامٍ مِنَ الشَّهْرِ  

"Uthman bin Abi Al-As" and he narrated something similar in Mursal form.  

عُثْمَانُ بْنُ أَبِي الْعَاصِ نَحْوَهُ مُرْسَلٌ  

"I heard Ibn 'Umar say" "The Prophet used to fast three days of each month.  

سَمِعْتُ ابْنَ عُمَرَ يَقُولُ كَانَ النَّبِيُّ ﷺ يَصُومُ ثَلاَثَةَ أَيَّامٍ مِنْ كُلِّ شَهْرٍ