Yahya bin Ayoub al-Ghafiqi يحيى بن أيوب الغافقي · Fasting (Kitab Al-Siyam)

When the Prophet (ﷺ) came to Medina, he found the Jews observing fast on the day of 'Ashurah; so they were asked about it (by the Prophet). They said: This is a day on which Allah gave Moses domination over Pharaoh. We fast on it out of reverence to him. The Messenger of Allah (ﷺ) said: We have a closer connection with Moses than you have. He then gave orders that it should be observed.

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.