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
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.