Mishkat al-Masabih 2070
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.
وَعَنْ حَفْصَةَ قَالَتْ: أَرْبَعٌ لَمْ يَكُنْ يَدَعُهُنَّ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «صِيَامُ عَاشُورَاءَ وَالْعَشْرِ وَثَلَاثَةُ أَيَّامٍ مِنْ كُلِّ شَهْرٍ وَرَكْعَتَانِ قبل الْفجْر» . رَوَاهُ النَّسَائِيّ