"The Messenger of Allah used to fast Thursday and Monday of each month, and Monday of the following week." (Using translation from Nasāʾī 2366)
كَانَ رَسُولُ اللهِ ﷺ يَصُومُ ثَلَاثَةَ أَيَّامٍ مِنْ كُلِّ شَهْرٍ يَوْمَ الْخَمِيسِ وَيَوْمَ الِاثْنَيْنِ وَمِنَ الْجُمُعَةِ الثَّانِيَةِ يَوْمَ الِاثْنَيْنِ