Sayyidah Hafsah (may Allah be pleased with her) said: There were four things which the Prophet (peace and blessings be upon him) never abandoned: the fast of Ashura, the fast of the ten days (of Dhu al-Hijjah), and the fast of three days of every month.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19700