Sahih Muslim 1160
Sahih Muslim · Book 13: The Book of Fasting · 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
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:
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 .
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.