Voluntary Fasting - Section 3 Mishkat al-Masabih 2070
Translation
Hafsa said there were four things the Prophet never omitted
fasting on ‘Ashura’, the first ten days of Dhul Hijja* and three days every month, and praying two rak'as before dawn. *The Arabic has simply 'the ten', to be understood as translated above.Nasa’i transmitted it.