Muwatta Malik
Book 18, Hadith 35
Arabic hadith text
وَحَدَّثَنِي عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ أَنَّ عُمَرَ بْنَ الْخَطَّابِ، أَرْسَلَ إِلَى الْحَارِثِ بْنِ هِشَامٍ أَنَّ غَدًا، يَوْمُ عَاشُورَاءَ فَصُمْ وَأْمُرْ أَهْلَكَ أَنْ يَصُومُوا .
Translation
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)."