I asked 'Aishah: Would the Messenger of Allah ﷺ 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. (Using translation from Abū Dāʾūd 2453)   

عَائِشَةَ ؓ أَكَانَ رَسُولُ اللهِ ﷺ يَصُومُ مِنْ كُلِّ شَهْرٍ ثَلَاثَةَ أَيَّامٍ؟ قَالَتْ نَعَمْ قُلْتُ مِنْ أِيِّ أَيَّامِ الشَّهْرِ كَانَ يَصُومُ؟ قَالَتْ مَا كَانَ يُبَالِي مِنْ أِيِّ الشَّهْرِ كَانَ يَصُومُ