Muwatta Malik 666
موطأ مالك#666Da'ifYahya 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)
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Salim al-HilaliMauquf Daif
In-book reference: 18:37