the Messenger of Allah used to fast three days of each month: Monday at the beginning of the month, the following Thursday, then the Thursday after that. (Using translation from Nasāʾī 2414)  

أَنَّ رَسُولَ اللهِ ﷺ كَانَ «يَصُومُ ثَلَاثَةَ أَيَّامٍ مِنْ كُلِّ شَهْرٍ يَوْمَ الِاثْنَيْنِ مِنْ أَوَّلِ الشَّهْرِ ثُمَّ الْخَمِيسِ الَّذِي يَلِيهِ ثُمَّ الْخَمِيسِ الَّذِي يَلِيهِ»  

"I entered upon the Mother of the Believers and heard her say: 'The Messenger of Allah used to fast three days of each month: The first Monday of the month, then Thursday, then the following Thursday." (Using translation from Nasāʾī 2415)  

سَمِعْتُ هُنَيْدَةَ الْخُزَاعِيَّ يَقُولُ دَخَلْتُ عَلَى أُمِّ الْمُؤْمِنِينَ سَمِعْتُهَا تَقُولُ كَانَ رَسُولُ اللهِ ﷺ يَصُومُ مِنْ كُلِّ شَهْرٍ ثَلَاثَةَ أَيَّامٍ أَوَّلَ اثْنَيْنِ مِنَ الشَّهْرِ ثُمَّ الْخَمِيسَ ثُمَّ الْخَمِيسَ الَّذِي يَلِيهِ  

"There are four things which the Prophet never gave up: Fasting 'Ashura', (fasting during) the ten days, (fasting) three days of each month, and praying two Rak'ahs before Al-Ghadah (Fajr)." (Using translation from Nasāʾī 2416)  

أَرْبَعٌ لَمْ يَكُنْ يَدَعُهُنَّ النَّبِيُّ ﷺ «صِيَامُ عَاشُورَاءَ وَالْعَشْرُ وَثَلَاثَةُ أَيَّامٍ مِنْ كُلِّ شَهْرٍ وَرَكْعَتَينِ قَبْلَ الْغَدَاةِ»  

the Messenger of Allah used to fast nine days of Dhul-Hijjah, the day of 'Ashura', and three days of each month: The first Monday of the month, and two Thursday. (Using translation from Nasāʾī 2417)  

عَنْ بَعْضِ أَزْوَاجِ النَّبِيِّ ﷺ أَنَّ رَسُولَ اللهِ ﷺ كَانَ «يَصُومُ تِسْعًا مِنْ ذِي الْحِجَّةِ وَيَوْمَ عَاشُورَاءَ وَثَلَاثَةَ أَيَّامٍ مِنْ كُلِّ شَهْرٍ أَوَّلَ اثْنَيْنِ مِنَ الشَّهْرِ وَخَمِيسَيْنِ»  

"The Prophet used to fast the ten (days), and three days of each month: Monday and Thursday." (Using translation from Nasāʾī 2418)  

عَنْ بَعْضِ أَزْوَاجِ النَّبِيِّ ﷺ قَالَتْ كَانَ رَسُولُ اللهِ ﷺ «يَصُومُ الْعَشْرَ وَثَلَاثَةَ أَيَّامٍ مِنْ كُلِّ شَهْرٍ الِاثْنَيْنِ وَالْخَمِيسَينِ»  

"The Messenger of Allah used to enjoin fasting three days: The first Thursday, and Monday and Monday." (Using translation from Nasāʾī 2419)  

كَانَ رَسُولُ اللهِ ﷺ «يَأْمُرُ بِصِيَامِ ثَلَاثَةِ أَيَّامٍ أَوَّلَ خَمِيسٍ وَالِاثْنَيْنِ وَالِاثْنَيْنِ»  

"Fasting three days of each month is fasting for a lifetime, and the shining days of Al-Bid, the thirteenth, fourteenth and fifteenth." (Using translation from Nasāʾī 2420)  

عَنِ النَّبِيِّ ﷺ قَالَ «صِيَامُ ثَلَاثَةِ أَيَّامٍ مِنْ كُلِّ شَهْرٍ صِيَامُ الدَّهْرِ وَأَيَّامُ الْبِيضِ صَبِيحَةَ ثَلَاثَ عَشْرَةَ وَأَرْبَعَ عَشْرَةَ وَخَمْسَ عَشْرَةَ»