"The Messenger of Allah used to fast three days of every month: the Monday and Thursday of the first week and the Monday of the following week." (Using translation from Nasāʾī 2365)
كَانَ رَسُولُ اللهِ ﷺ يَصُومُ مِنْ كُلِّ شَهْرٍ ثَلَاثَةَ أَيَّامٍ الِاثْنَيْنِ وَالْخَمِيسَ مِنْ هَذِهِ الْجُمُعَةِ وَالِاثْنَيْنِ مِنَ الْمُقْبِلَةِ