Saad arrived with his force of 4,000 at Zarud and went into camp. The troops were spread in the region, and couriers were sent to all the tribes in Northern Arabia calling the tribesmen to war in the name of Allah. As a result of these efforts about 7,000 warriors were recruited from the tribes particularly the Bani Asad and Bani Tameem. Among those who joined the Muslim forces was Taleaha who had during the apostasy campaigns of the time of Abu Bakr claimed to be a prophet and had fought against the Muslims. He had escaped to Syria where he was converted to Islam. Those who had once apostated were not allowed by Abu Bakr to be recruited to the Muslim forces. Hazrat Umar, because of the large scale campaigns to be undertaken lifted the ban. Availing of this concession Taleaha and his tribesmen joined the Muslim forces in Iraq in large numbers.
Umar sent another force of 4,000 men to join the main army at Zarud. The strength of the army at Zarud now rose to 15,000. Muthanna with 3,000 men was stationed at Sharaf some sixty miles from Zarud on the main route to Iraq.
From Zarud the main Muslim army marched to Sharaf, and they arrived there in July 636 A.D. Before Saad arrived, Muthanna was dead. In his will Musanna had desired that Saad bin Abi Waqas should marry his widow Salma bint Khasfa. He also left a message for Saad which ran as follows:
"The Muslims should not fight the Persians when they are concentrated in their homeland, but should fight them on the boundary near the desert. Thus if Allah should give the Muslims victory, they will have whatever lies behind the Persians, and if the result is otherwise, they can withdraw into a region the routes whereof they know best and of which they are masters-until Allah decides that they should return to battle."
Saad prayed for the soul of Muthanna. He paid rich tributes to his bravery. In fulfilment of the will of Muthanna, Saad married his widow Salma. He was impressed with the parting advice of Muthanna, and decided to follow it. He reported this advice to Umar who approved of it.
Umar instructed Saad as follows:
"Organise the army into tens and let the men know their units.
Appoint the commanders of the corps and let them see and know their men.
Give the contingents Qadisiyya as the meeting point.
Get Mugheera bin Shu'ba to join you with his cavalry.
Comply with these instructions and then write to me."
Saad organised the army in accordance with the instructions of Umar. From the Uballa sector Mugheera bin Shu'ba joined Sad with his cavalry of 800 horse.
Umar next instructed Saad as follows:
"March with the Muslims from Sharaf towards the Persians.
Place your faith in Allah and seek His help, and know that you are advancing against a people whose numbers are vast, whose equipment is superb, whose strength is great and whose land is difficult. Even its plains consist of rivers and heavily-watered land. When you meet them or any of them, attack them fiercely, but beware of facing them if they are all together. Let them not trick you, for they are wily plotters and their ways are not your ways.
When you get to Qadisiyya, remain there and leave not your place. They will find your continued stay intolerable and will come out against you with all their strength of horse and foot. And if you stand fast against them, you shall overcome them, and should they ever assemble again in great numbers, they shall do so without hearts.
And should the result be otherwise, you will have the desert behind you and can withdraw into a region which you know and control and of which they are ignorant and afraid. And there you should stay until Allah decides victory for you and you return to battle."