(36) باب اسْتِحْبَابِ صِيَامِ ثَلَاثَةِ أَيَّامٍ مِنْ كُلِّ شَهْرٍ وَصَوْمِ يَوْمِ عَرَفَةَ وَعَاشُورَاءَ وَالِاثْنَيْنِ وَالْخَمِيسِ
'Imran b. Husain (Allah be pleased with them) reported that the Messenger of Allah (ﷺ) said to him (or he said to another person and he was listening to it):
O, so and so, did you observe fast in the middle of the month? He said: No. Thereupon he (the Messenger of Allah) said: When you break it, then observe fast for two days.
Reference: Sahih Muslim 1161 a
In-book reference: Book 13, Hadith 251
USC-MSA web (English) reference(deprecated numbering scheme): Book 6, Hadith 2601