The apostle of Allah (ﷺ) used to fast three days every month: Monday, Thursday and Monday in the next week.
Sunan Abi Dawud Book 14, Hadith 139
← Previous Hadith The Messenger of Allah (ﷺ) used to fast three days every month.

Sunan Abi Dawud Book 14, Hadith 138 Next 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