Mishkat al-Masabih 7 Fasting كتاب الصوم 89

وَعَنْ مُعَاذَةَ الْعَدَوِيَّةِ أَنَّهَا سَأَلَتْ عَائِشَةَ: أَكَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَصُومُ مِنْ كُلِّ شَهْرٍ ثَلَاثَةَ أَيَّامٍ؟ قَالَتْ: نَعَمْ فَقُلْتُ لَهَا: مِنْ أَيِّ أَيَّامِ الشَّهْرِ كَانَ يَصُومُ؟ قَالَتْ: لَمْ يَكُنْ يُبَالِي مِنْ أَيِّ أَيَّام الشَّهْر يَصُوم. رَوَاهُ مُسلم

Mu’adha al-‘Adawiya said she asked ‘A’isha whether God’s messenger used to fast three days every month, and she replied that he did. She asked which days in the month he fasted, and she replied that he did not care which days of the month he fasted. Muslim transmitted it.