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 1160

Mu'adha al-'Adawiyya reported that she asked 'A'isha, the wife of the Messenger of Allah (ﷺ), whether the Messenger of Allah (ﷺ) observed fasts for three days during every month. She said:

Yes I said to her: Which were (the particular) days of the month on which he observed fast? She said: He was not particular about the days of the month on which to observe fast.
حَدَّثَنَا شَيْبَانُ بْنُ فَرُّوخَ، حَدَّثَنَا عَبْدُ الْوَارِثِ، عَنْ يَزِيدَ الرِّشْكِ، قَالَ حَدَّثَتْنِي مُعَاذَةُ، الْعَدَوِيَّةُ أَنَّهَا سَأَلَتْ عَائِشَةَ زَوْجَ النَّبِيِّ صلى الله عليه وسلم أَكَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يَصُومُ مِنْ كُلِّ شَهْرٍ ثَلاَثَةَ أَيَّامٍ قَالَتْ نَعَمْ ‏.‏ فَقُلْتُ لَهَا مِنْ أَىِّ أَيَّامِ الشَّهْرِ كَانَ يَصُومُ قَالَتْ لَمْ يَكُنْ يُبَالِي مِنْ أَىِّ أَيَّامِ الشَّهْرِ يَصُومُ ‏.‏
Reference : Sahih Muslim 1160
In-book reference : Book 13, Hadith 250
USC-MSA web (English) reference : Book 6, Hadith 2600
  (deprecated numbering scheme)