In July 6.36 A.D. the main Muslim army marched from Sharaf to Qadisiyya. Qadisiyya was on the west bank of the Ateeq, a branch of the Euphrates. It was the last staging camp in Arabia on the route to Iraq. Hira lay about thirty miles ahead.
After establishing the camp, organising the defences, and securing the river heads, Saad sent parties inside the Suwad to conduct raids. In one of these raids the Muslims captured the bridal procession of the daughter of Azazbeh the Persian Governor of Hira. A large booty was captured including the bride and other Persian damsels.
From Qadisiyya, Saad wrote to Umar:
"The enemy has sent no one against us and has not appointed, so far we know, anyone to command the campaign. When we get the information, we will report to you and seek Allah's help."
To this Umar replied in the following terms:
"Strengthen your heart and your army with sermons and right intentions and worthiness; and as for those who forget, remind them. Steadfastness and again steadfastness! For help comes from Allah according to one's intentions and His reward according to one's deserts. Caution and again caution! For grave is the matter upon which you are embarked. And pray to Allah for his blessiogs.
Write to me when you know of the concentration of their army and who commands it, for in the absence of knowledge about their army and its commander I am not able to give you much guidance.
Describe the place where you are and the land between you and Medina. Describe it so clearly that I may see it with my own eyes, and become one of you.
Fear Allah and in Him rest your hopes."
Saad sent the required topographical information. The intelligence reports of the spies were also forwarded to Umar. Saad stated that all the people of the Suwad who had entered into pacts with the Muslims had gone back on their pledges. They were collaborating with the Persians and were preparing for war against the Muslims.
"The Commander of the Persian army is Rustam, and he has some other top-ranking Generals.
They seek to weaken us and pounce upon us and we seek to weaken them and attack them. The command of Allah is as good as bone and His decision will be according to whatever He wishes for us or against us. We beseech Allah for the best of decisions and the best of judgements." In reply, Umar instructed:
"Remain where you are until Allah fixes your enemy for you.
And if Allah should give you victory, pursue them until you fall upon Madain, which Allah willing will be destroyed."
A week later, Umar further instructed:
"My heart tells me that when you meet the enemy you will God willing defeat him. So dispel all doubt from your mind, and if any of you gives a promise of peace to a Persian, with sign or speech, even if he does not understand it, let him fulfil the promise.
Beware of jesting, faithfulness and again faithfulness. Errors committed in good faith are acceptable but deliberate unfaithfulness leads to destruction. In it will lie your weakness and your enemy's strength, the depression of your courage and the elevation of the courage of your enemy.
I caution you not to bring dishonour to the Muslims, nor be a cause of their disgrace."
Saad reported about the large strength of the forces of the Persians. To this Umar replied as follows:
"Let not the information which you get about the enemy distress you. Seek Allah's help, and in Him place your trust."
Umar also desired that a delegation of some intelligent persons should be sent to the emperor of Persia to call him to Islam.
After the battle of Yermuk some forces were released from the Syrian front and sent to Iraq under the command of Ath'ath bin Qais. The strength of Saad's army at Qadisiyya now rose to 29,000.
The Muslim forces intensified their raiding activities. The entire Suwad now became a hunting ground for the Muslim raiders. These raids were undertaken partly to gather supplies for the Muslim army, and partly to demoralise the Persians.
The inhabitants of the Suwad appealed to the Persian emperor to do something urgently to save them from the raids of the Muslims. The emperor Yazdjurd assured them that he was sending a large force under Rustam against the Muslims, and he would crush the Arabs.