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. (Using translation from Abū Dāʾūd 2437)  

عَنْ بَعْضِ أَزْوَاجِ النَّبِيِّ ﷺ قَالَتْ كَانَ رَسُولُ اللهِ ﷺ يَصُومُ تِسْعَ ذِي الْحِجَّةِ وَيَوْمَ عَاشُورَاءَ وَثَلَاثَةَ أَيَّامٍ مِنْ كُلِّ شَهْرٍ أَوَّلَ اثْنَيْنِ مِنَ الشَّهْرِ وَخَمِيسَيْنِ