Hunaydah ibn Khaalid narrated upon the authority of his wife who said, “Some of the wives of the Prophet صلى الله عليه وسلم told me that the Prophet صلى الله عليه وسلم used to fast the Day of ‘Aashoorah, the first nine days of Dhul-Hijjah, and three days out of every month (13th, 14th, and 15th every month)”

Imam Ahmad, An-Nasaaee