عَنْ حَفْصَةَ زَوْجِ النَّبِيِّ رَضِيَ اللَّهُ عَنْهَا أَنَّ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَآلِهِ وَسَلَّمَ كَانَ يَصُومُ ثَلَاثَةَ أَيَّامٍ مِنْ كُلِّ شَهْرٍ يَوْمَ الْإِثْنَيْنِ وَيَوْمَ الْخَمِيسِ وَيَوْمَ الْإِثْنَيْنِ مِنَ الْجُمُعَةِ الْأُخْرَى
It is narrated from Sayyida Hafsa (may Allah be pleased with her) that the Noble Prophet (peace and blessings be upon him and his family) used to fast three days every month: (in the first week) on Monday and Thursday, and in the second week on Monday.