Sunan Abi Dawud, Hadith 2447 ⓘ
Mu'adhah (al-'Adawiyyah) said: I asked 'Aishah: Would the Messenger of Allah fast three days every month ? She replied: Yes. I asked: Which days in the month he used to fast ? She replied: He did not care which days of the month he fasted.
حَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا عَبْدُ الْوَارِثِ، عَنْ يَزِيدَ الرِّشْكِ، عَنْ مُعَاذَةَ، قَالَتْ قُلْتُ لِعَائِشَةَ أَكَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يَصُومُ مِنْ كُلِّ شَهْرٍ ثَلاَثَةَ أَيَّامٍ قَالَتْ نَعَمْ . قُلْتُ مِنْ أَىِّ شَهْرٍ كَانَ يَصُومُ قَالَتْ مَا كَانَ يُبَالِي مِنْ أَىِّ أَيَّامِ الشَّهْرِ كَانَ يَصُومُ .
also referenced under: Hadith 2453 (DarusSalam)