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.  

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  

The Messenger of Allah ﷺ as saying: There is no virtue more to the liking of Allah in any day than in these days, that is, the first ten days of Dhu al-Hijjah. They (the Companions) asked: Messenger of Allah, not even the struggle in the path of Allah (Jihad) ? He said: (Yes), not even the struggle in the path of Allah, except a man who goes out (in the path of Allah) with his life and property, and does not return with any of them.  

قَالَ رَسُولُ اللَّهِ ﷺ مَا مِنْ أَيَّامٍ الْعَمَلُ الصَّالِحُ فِيهَا أَحَبُّ إِلَى اللَّهِ مِنْ هَذِهِ الأَيَّامِ يَعْنِي أَيَّامَ الْعَشْرِ قَالُوا يَا رَسُولَ اللَّهِ وَلاَ الْجِهَادُ فِي سَبِيلِ اللَّهِ قَالَ وَلاَ الْجِهَادُ فِي سَبِيلِ اللَّهِ إِلاَّ رَجُلٌ خَرَجَ بِنَفْسِهِ وَمَالِهِ فَلَمْ يَرْجِعْ مِنْ ذَلِكَ بِشَىْءٍ