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

English translation · Sahih Muslim