"I entered upon the Mother of the Believers and heard her say: 'The Messenger of Allah used to fast three days of each month: The first Monday of the month, then Thursday, then the following Thursday." (Using translation from Nasāʾī 2415)
سَمِعْتُ هُنَيْدَةَ الْخُزَاعِيَّ يَقُولُ دَخَلْتُ عَلَى أُمِّ الْمُؤْمِنِينَ سَمِعْتُهَا تَقُولُ كَانَ رَسُولُ اللهِ ﷺ يَصُومُ مِنْ كُلِّ شَهْرٍ ثَلَاثَةَ أَيَّامٍ أَوَّلَ اثْنَيْنِ مِنَ الشَّهْرِ ثُمَّ الْخَمِيسَ ثُمَّ الْخَمِيسَ الَّذِي يَلِيهِ