Narrated One of the wives of the Prophet:

Hunaydah ibn Khalid narrated from his wife on the authority of one of the wives of the Prophet ﷺ who said: The Messenger of Allah ﷺ used to fast the first nine days of Dhul-Hijjah, Ashura' and three days of every month, that is, the first Monday (of the month) and Thursday.

حَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا أَبُو عَوَانَةَ، عَنِ الْحُرِّ بْنِ الصَّبَّاحِ، عَنْ هُنَيْدَةَ بْنِ خَالِدٍ، عَنِ امْرَأَتِهِ، عَنْ بَعْضِ، أَزْوَاجِ النَّبِيِّ ﷺ قَالَتْ كَانَ رَسُولُ اللَّهِ ﷺ يَصُومُ تِسْعَ ذِي الْحِجَّةِ وَيَوْمَ عَاشُورَاءَ وَثَلاَثَةَ أَيَّامٍ مِنْ كُلِّ شَهْرٍ أَوَّلَ اثْنَيْنِ مِنَ الشَّهْرِ وَالْخَمِيسَ .