SHAFAQNA – Yazid ibn Moawiyeh ordered Ubaidullah ibn Ziad to fight with Imam Hussain (AS) if Imam (AS) did not swear allegiance to Yazid. Before the events of Karbala, Omar ibn Saad had been appointed as the governor of Rey by Ubaidullah. But before Omar taking charge, Ubaidullah wrote a letter to him saying, Imam Hussain (AS) has come to Iraq, you must go to Iraq, fight with him (Imam Hussain (AS)) and kill him, and then go to Rey. Omar ibn Saad came to see Ubaidullah and said: O’ Amir; excuse me from this task. Ubaidullah said: I will Excuse you, but I will take away the governorship of Rey from you. Omar became hesitant between fighting Imam (AS) and the governorship of Rey and said: Give me one night to think about this task and Ubaidullah gave him time to think. Omar thought about the governorship of Rey till morning and eventually decided to take the governorship and fight with Imam Hussain (AS). The next morning, Omar went to see Ubaidullah and accepted to kill Imam Hussain (AS) and Ubaidullah prepared a large army for this purpose. Imam Hussain (AS) came to karbala on the second day of Muharram, and Omar ibn Saad entered Karbala on the third day of Muharram and became the chief commander of the army, and appointed Shemr as the commander. For the sake of the governorship of Rey, on the tenth day of Muharram, Omar ibn Saad ordered Yazid’s army to kill Imam Hussain (AS) and 72 of his children and companions .
 Montahal Amaal, Vol.1, Page 333.