Maliks Muwatta Book 18, Hadith Number 35.
Section : Fasting on the Day of Ashura (the 10th of Muharram).
Yahya related to me from Malik that he had heard that Umar ibn al-Khattab had sent (the following message) to al-Harith ibn Hisham, ”Tomorrow is the day of Ashura, so fast (it) and tell your family to fast (also).”