Sayyidah Hafsah (may Allah be pleased with her) says: There were four things which the Prophet (peace and blessings be upon him) never abandoned: the fast of Ashura, fasting the ten days (of Dhul-Hijjah), fasting three days of every month, and the two rak'ahs before Fajr.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19658