Sahih Muslim #2744 · The Book of Fasting

Sahih MuslimMu'adha al-'Adawiyya reported that she asked 'A'isha
Sahih Muslim · Book 13 · The Book of Fasting
Sahih Muslim · كتاب 13 · كتاب الصيام
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 2744In-book reference : Book 13, Hadith 250