In such day, on the ninth of Muharram 61 AH, the cursed Shimr arrived to Karbala carrying a letter from Ubaydullah Ibn Ziyad to Omar Ibn Sa'd, stating: "I did not send you to al-Hussayn for you to restrain yourself from [fighting] him, nor to idle the time away with him, nor to promise him peace and preservation [of his life], nor to be an intercessor on his behalf with me. Look now, if al-Hussayn and his companions submit to [our] authority and surrender, then send them to me as captives. But if they refuse, then march against them until you kill them. Treat them in a manner that it should serve as a lesson for others, for they indeed deserve that!
When al-Hussayn is killed, make the horses trample on his chest and back; for he is disobedient and an opponent, an oppressor and one who is set to sever [all] relations. I do not consider that this [i.e. trampling his body] will be in any way wrong after death. But I have promised myself that I would do this to him if I killed him! If you carry out our command with respect to him, we will reward you the reward of one who is obedient. [But] if you refuse, then leave our service and army and hand it over to Shimr Ibn Dhi al-Jawshan; for we have given him our instructions. That is all.”
When he arrived and read it to him ‘Umar said: “Woe onto you! What is wrong with you? May Allah never show favour to your house. May Allah make abominable what you have brought to me? By Allah! I suspect you to have dissuaded him from accepting what I wrote to him. You ruined for us a matter which we had hoped to set right. By Allah, al-Hussayn will not surrender. An unsubmissive soul exists within him!”
Shimr said to him: “Tell me what you are going to do. Are you going to carry out the instructions of your governor and kill his enemy? Otherwise leave the command of the army to me.”
[Ibn Sa‘d] replied: “No! There is no honour for you! I will carry that out. Here you are! Take control of the foot soldiers!”…
It is narrated that Imam as-Sadeq (peace be upon him) said: "Tasu'a [ninth of Muharram] is the day when al-Hussayn (peace be upon him) and his companions were besieged in Karbala and the army of Syria gathered against them; and 'Ubaydullah Ibn Ziyad and Omar Ibn Sa'd became so happy of the gathering of so many forces. That day, they considered al-Hussayn (peace be upon him) and his companions weak and became sure that no help will come for them and Iraqi people will not support them either."
He (the Imam) then said, ‘The day of ‘Ashura’ is the day on which al-Hussayn (peace be upon him) was murdered and his friends around him were all murdered and looted. How can there be fast on that day! No, by the Lord of the sacred house there is no fasting on such a day. It is only a day of mourning and lamentation that came upon the inhabitants of the sky and earth and all believing people. It is a day of happiness and joy for ibn Marjanah and family of Ziyad and people of al-Sham, may Allah subject them to His anger as well as their offspring. This was a day when all locations of earth wept except al-Sham.