(36) باب اسْتِحْبَابِ صِيَامِ ثَلَاثَةِ أَيَّامٍ مِنْ كُلِّ شَهْرٍ وَصَوْمِ يَوْمِ عَرَفَةَ وَعَاشُورَاءَ وَالِاثْنَيْنِ وَالْخَمِيسِ
Abu Qatada Ansari (Allah be pleased with him) reported that Allah's Massenger (ﷺ) was asked about fasting on Monday, whereupon he said:
It is (the day) when I was born and revelation was sent down to me.
Reference: Sahih Muslim 1162 e
In-book reference: Book 13, Hadith 256
USC-MSA web (English) reference(deprecated numbering scheme): Book 6, Hadith 2606