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.
Sunan Abi Dawud Book 14, Hadith 141
← Previous Hadith Hunaydah al-Khuza'i reported on the authority of her mother who said: I entered upon Umm Salamah and asked her about fasting. She said: The Messenger of Allah (ﷺ) used to command me to fast three days every month beginning with Monday or Thursday.

Sunan Abi Dawud Book 14, Hadith 140 Next Hadith → The Messenger of Allah (ﷺ) said: He who does not determine to fast before dawn does not fast.

Sunan Abi Dawud Book 14, Hadith 142