The Persians crossed the Ateeq on the 16th November, 636 A.D. The previous night Rustam had a dream in which he had seen Umar seal the arms of the Persians. As Rustam woke he said to himself: "This Umar has eaten my heart. May God burn him."
As Rustam saw his warriors cross the river and take up their positions for battle, he felt over-confident. He remarked to an Officer, "With this army we will shatter the Arabs into pieces." The Officer added the words, "If God wills it." Rustam was in a defiant mood and he retorted, "Even if He does not will it."
The Persian army was deployed with five corps holding the front and one corps in reserve, each corps having a depth of 13 ranks. The centre was commanded by Rustam himself. The other Commanders were: Left Centre: Beerzan; Right Centre: Jalinus; Left Wing: Mihran; and Right Wing: Hormuzan. The reserve was commanded by Bahman.
The Persian army had a strength of 60,000 men. There were 33 war elephants in the Persian army each mounted by several men armed with javelins and bows.
At an elevated seat shaded hy a canopy near the west bank of the river sat Rustam wearing his armour. By his side waved the Dirafashe-Kavian the standard of the Persians.
The Muslim Commander-in-Chief Saad b. Abi Waqas was suffering from sciatica, and there were boils all over his body. In Qadisiyya there was an old royal palace which stood at the extremity of the battle-field. Saad took a seat in the upper storey of the palace where he lay propped up by pillows. From this seat he directed the war operations. He appointed Khalid b. Arfatah as his Deputy, who maintained liasion with the army, and carried out whatever instructions were issued by Saad from time to time.
In the centre of the Muslim army the infantry was commanded by Hammal b. Malik. The other Commanders were: Left Centre: Asim bin Amr; Right Centre: Zuhra b. Al-Hawiyya; Left Wing: Shurahbeel b. As-Samt; Right Wing: Abdullah b. Al-Mut'im. The reserve was commanded by Salman bin Rabee'a.
When the Muslim forces were arrayed in the order of battle, poets and orators addressed them, and with their stirring declamations inspired the warriors to action.
One of the orators said;
"O warriors, turn your swords into an impenetrable wall of steel; rush upon your antagonists like so many roaring lions; don the panoply of dust and turn your eyes downwards. When you have done with swords then let the arrows fly, for the swords cannot reach where arrows find their way."
Readers of the Quran recited verses from the Holy Quran on the subject of 'Jihad' with forceful cadence which stirred the hearts of the warriors.
The battle began with personal duels. The first duel was between Ghalib b. Abdullah of the Bani Asad and the Persian General Hormuz. Hormuz was overpowered and brought to the Muslim camp where he was locked as a prisoner of war. From the Persian ranks a Bersian officer stepped forward and gave a challenge. This was accepted from the Muslim ranks by Amr b. Mndi Karib. They wrestled for some time when Amr threw his adversary and cut off his head Amr then turned to his men and shouted: " When a Persian has dropped his javelin he is useless". Then another Persian stepped forward. Amr closed up and lifted the Persian off his horse, and then cut his throat. Then he shouted, "When a Persian has lost his bow, he is useless". The Arab champion returned to his ranks, and turning to his companions shouted, "I am Abu Thaur. Do as I do." To this his admiring companions replied: "O Father of the Bull, who can do as you do."
There was another combat between Asim b Amr and a Persian Officer. When the Persian got near Asim, he lost nerves, and galloped back to the Persian army. Asim followed him to the Persian line, but no Persian stepped forward to meet the challenge of Asim. Asim found a mule loaded with two saddle bags. Asim took the reins of the mule and led it to the Muslim camp. The saddle bags were found full of date cakes and honey. Saad gifted this trophy to the men belonging to the contingent of Asim.
After the duels were over, Rustam struck at the Muslims with his elephants and his wings. The Persians attack began with heavy showers of arrows. The Muslim archers shot their arrows in return, but these were light, and the Persians derisively said that the Muslim arrows were mere spindles.
After the Persian archers had gained the upper hand, Rustam ordered an attack on the Muslim right. The elephants led the attack and advanced upon the contingent led by Jareer b. Abdullah. As the elephants advanced, the Muslim horses broke out of control and fled from their position thereby leaving the infantry unsupported. As the elephants advanced the Muslim infantry was thrown into confusion, and began to fall back.
Saad seated upstairs in the Qadisiyya palace saw this confusion. He was writhing in an agony of pain, and was impatiently tossing from side to side. His wife Salma the widow of Muthanna was seated close to him. Seeing the confusion in the Muslim ranks Salma exclaimed, "What a pity, Muthanna is not here to-day." Cut to the quick, Saad slapped her on the face saying, 'What could have Muthanna done even if he were present?" Salma retorted "I wonder the cowards have also a sense of honour". Then she walked away inside the house.
Saad sent orders to Ath'ath b. Qais who commanded the Kinda in the right centre, and to Hammal b. Malik who commanded the infantry of the centre to attack the Persian corps which were pursuing the Bajeela contingent. Using javelin and sword the Muslims arrested the Persian advance. Then the advancing Persians were attacked from the front as well as the flank. That made the Persians withdraw.
Rustam now ordered his right wing under Jalinus to advance against the Muslims on their front. The elephants of the Persian right and right centre moved forward. The Persian archers came into action and let loose a rain of arrows. The Muslim horses on the left and centre became unmanageable and fled from their positions. Saad sent word to Asim b. Amr who commanded the Bani Tamim to do something about the elephants.
Asim ordered his men to pick off the Persians on the elephants backs with arrows, to get behind the elephants and then slip in and cut the girths of the howdahs. Bani Tamim rushed forward to their task, and soon the girths of all the howdahs had been cut. Many Persian elephant-riders were killed as they fell, and the rest beat a hasty retreat making the elephants retire to the position behind the front line.
By afternoon the Persian attacks on the Muslim wings were beaten back. Now Saad ordered a counter attack. The Muslim front at once moved forward. The Muslim cavalry charged with full force. That made the Persians reel back. The Muslim infantry then advanced. The javelin-men hurled their javelins, and the swordsmen rushed forward brandishing their swords. About sunset the Muslims were able to create several gaps in the Persian front. Through one of such gaps the Muslim warriors charged and got very near Rustam the Persian Commander-in-Chief. Rustam plunged into the foray personally and repulsed the Muslim attack though he received several wounds on his person.
The fighting ended at dusk. The battle was inconclusive. There was considerable confusion and loss on both the sides. In the Muslim chronicles the first day of the battle of Qadisiyya came to be known as "The Day of Disorder."