The Messenger of Allah ﷺ used to command us to fast the days of the white (nights): thirteenth, fourteenth and fifteenth of the month. He said: This is like keeping perpetual fast.  

كَانَ رَسُولُ اللَّهِ ﷺ يَأْمُرُنَا أَنْ نَصُومَ الْبِيضَ ثَلاَثَ عَشْرَةَ وَأَرْبَعَ عَشْرَةَ وَخَمْسَ عَشْرَةَ قَالَ وَقَالَ هُنَّ كَهَيْئَةِ الدَّهْرِ  

The Messenger of Allah ﷺ used to fast three days every month.  

كَانَ رَسُولُ اللَّهِ ﷺ يَصُومُ يَعْنِي مِنْ غُرَّةِ كُلِّ شَهْرٍ ثَلاَثَةَ أَيَّامٍ