The apostle of Allah ﷺ used to fast three days every month: Monday, Thursday and Monday in the next week.  

كَانَ رَسُولُ اللَّهِ ﷺ يَصُومُ ثَلاَثَةَ أَيَّامٍ مِنَ الشَّهْرِ الاِثْنَيْنِ وَالْخَمِيسِ وَالاِثْنَيْنِ مِنَ الْجُمُعَةِ الأُخْرَى  

Hunaydah al-Khuza'i reported on the authority of her mother who said: I entered upon Umm Salamah and asked her about fasting. She said: The Messenger of Allah ﷺ used to command me to fast three days every month beginning with Monday or Thursday.  

كَانَ رَسُولُ اللَّهِ ﷺ يَأْمُرُنِي أَنْ أَصُومَ ثَلاَثَةَ أَيَّامٍ مِنْ كُلِّ شَهْرٍ أَوَّلُهَا الاِثْنَيْنِ وَالْخَمِيسِ