Salim bin 'Abdullah bin 'Umar سالم بن عبد الله بن عمر بن الخطاب · Fasting (Kitab Al-Siyam)

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.