Yahya related to me from Malik that he had heard that Umar ibnal-Khattab had sent (the following message) to al-Harith ibn Hisham,''Tomorrow is the day of Ashura, so fast (it) and tell your family tofast (also)." (IT)
USC-MSA web (English) reference: Book 18, Hadith 35 (IT)
Arabic reference (IT): Book 18, Hadith 669 (IT)