(36) باب اسْتِحْبَابِ صِيَامِ ثَلَاثَةِ أَيَّامٍ مِنْ كُلِّ شَهْرٍ وَصَوْمِ يَوْمِ عَرَفَةَ وَعَاشُورَاءَ وَالِاثْنَيْنِ وَالْخَمِيسِ
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.
Reference: Sahih Muslim 1160
In-book reference: Book 13, Hadith 250
USC-MSA web (English) reference(deprecated numbering scheme): Book 6, Hadith 2600