Narrated One of the wives of the Prophet: Hunaydah ibn Khalid narrated from his wife on the authority of one of the wives of the Prophet (ﷺ) who said: The Messenger of Allah (ﷺ) used to fast the first nine days of Dhul-Hijjah, Ashura' and three days of every month, that is, the first Monday (of the month) and Thursday

English translation · Sunan Abu Dawud