كتاب الصيام
13  
The Book of Fasting
(36)
Chapter: It is recommended to fast three days of every month, and to fast on the days of 'Arafah and 'Ashura', and to fast on Mondays and Thursdays
(36)
باب اسْتِحْبَابِ صِيَامِ ثَلَاثَةِ أَيَّامٍ مِنْ كُلِّ شَهْرٍ وَصَوْمِ يَوْمِ عَرَفَةَ وَعَاشُورَاءَ وَالِاثْنَيْنِ وَالْخَمِيسِ
Sahih Muslim 1161 a

'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.
وَحَدَّثَنِي عَبْدُ اللَّهِ بْنُ مُحَمَّدِ بْنِ أَسْمَاءَ الضُّبَعِيُّ، حَدَّثَنَا مَهْدِيٌّ، - وَهُوَ ابْنُ مَيْمُونٍ - حَدَّثَنَا غَيْلاَنُ بْنُ جَرِيرٍ، عَنْ مُطَرِّفٍ، عَنْ عِمْرَانَ بْنِ حُصَيْنٍ، - رضى الله عنهما - أَنَّصلى الله عليه وسلم قَالَ لَهُ أَوْ قَالَ لِرَجُلٍ وَهُوَ يَسْمَعُ ‏"‏ يَا فُلاَنُ أَصُمْتَ مِنْ سُرَّةِ هَذَا الشَّهْرِ ‏"‏ ‏.‏ قَالَ لاَ ‏.‏ قَالَ ‏"‏ فَإِذَا أَفْطَرْتَ فَصُمْ يَوْمَيْنِ ‏"‏ ‏.‏
Reference : Sahih Muslim 1161a
In-book reference : Book 13, Hadith 251
USC-MSA web (English) reference : Book 6, Hadith 2601
  (deprecated numbering scheme)