مُوَطَّأُ مَالِكٍ
Muwatta Malik 669
Fasting
وَحَدَّثَنِي عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ أَنَّ عُمَرَ بْنَ الْخَطَّابِ، أَرْسَلَ إِلَى الْحَارِثِ بْنِ هِشَامٍ أَنَّ غَدًا، يَوْمُ عَاشُورَاءَ فَصُمْ وَأْمُرْ أَهْلَكَ أَنْ يَصُومُوا .
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)."
Arabic/English book reference: Book 18, Hadith 35