Early in January 637, the Muslim advance guard under Zuhra reached Kusa. It was only ten miles from Ctesiphon. There was a sizable detachment of the Persian forces here commanded by Shahryar. He was a huge camel-like man, very haughty and arrogant.
As the two forces were deployed for action, Shahryar stepped out of the Persian ranks, and gave the challenge for a personal duel in very arrogant terms. He said:
"Who can dare come forward to measure strength with me. Verily, whoever comes to fight with me, comes to meet his death."
Zuhra the Commander of the Muslim forces said:
"I would have come to fight with you personally, but in view of your arrogance I am deputing a slave to fight you. "
Under the orders of Zuhra, Nail bin Ju'shan emerged from the Muslim ranks and proceeded to have a duel with Shahryar. As the two antagonists faced each other mounted on horseback, Shahryar saw that Nail was a lean thin man whom he could overthrow in no time.
The duel began. They first fought with lances, then with swords. Thereafter the two champions dismounted from the horses and a hand to hand fighting ensued. In such a fight, Shahryar in view of his bulk appeared to have an upper hand. Shahryar overthrew Nail, and was about to draw his dagger to kill Nail, when the Arab with his strong teeth crushed the thumb of Shahryar. That made Shahryar lose his balance. Nail rose immediately, and thrust his dagger in the body of Shahryar. The camel-like man fell on the ground dead.
Seeing their commander fall, the Persians lost nerve, and choosing discretion as the better part of valour, they withdrew to Ctesiphon. Thereafter the Muslims occupied Kusa. A deputation of the citizens waited on Zuhra, when the usual terms were offered to them, and they accepted them without argument.
After the victory, Zuhra stayed at Kusa for some time. In the meantime all the Muslim forces reached Kusa. Kusa was a place of historic importance. It was the place where Nimrod imprisoned the Prophet Abraham, and where he was thrown in burning fire, out of which he had emerged unharmed. The Muslims visited all these sites and offered prayers for the soul of Abraham Sa'ad wrote of the Muslim victories to Umar, and also about the sanctity of Kusa.
In reply Umar wrote:
"Just as the Prophet Abraham emerged out of the ordeal of fire unharmed, thus I have the faith that in the battle of truth that you are waging against the Persians, you will, with the grace of God, triumph. Now Al-Madain awaits you. Go ahead, and let the Muslim flag flutter over the palace of the Chosroes. May God bless you. Have faith in God, for such faith would give you the courage to fight against heavy odds."