After the disaster of the Bridge the Muslim army under Muthanna was stationed at Ulleis. Both Umar and Muthanna sent heralds and emissaries to all parts of Arabia inviting the Arabs to participate in the war against the Persians.

In response to this call volunteers came from all parts of Arabia. Makhnaf b. Salim the chief of the Azd tribe came with 700 horsemen. A contingent of a thousand men of the Banu Tameem came under the command of Hasin b. Mabid. Adi the son of the legendary Hatim Tai came with a large contingent of his tribesmen. Contingents also came from the tribes of Rabab, Banu Kinanah, Khath'am, Banu Hanzalah, and Banu Dabbah. The Christian Arabs of the tribes of Narmr and Taghlab also joined to reinforce the Muslim war effort. To the clan of Bajeela led by Jareer bin Abdullah, Umar offered an additional share of the booty, out of the Khums-the state share.

After having received reinforcements, Muthanna moved to Zu Qar a few miles south of Qadisiyya. When the Persians came to know of the preparations of the Muslims they decided to send a strong force against the Muslims fed by Mihran. Mihran had been in Arabia and was regarded as an expert in the Arabian way of war. The Persian army under .Mihran marched to the Euphrates and camped on the east bank opposite he site of modern Kufa.

Muthanna with the Muslim army advanced from Zu Qar, and arriving on the west bank of the Euphrates camped at Nakheila. At Nakheila a stream Buwaib took off from the Euphrates.

Mihran sent a message to Muthanna whether the Muslims would like to cross the Euphrates, or whether they would like the Persians to cross over to their side. The Muslims have had a bitter experience of crossing the river in the 'Battle of the Bridge', and so Muthanna said to the Persian emissary "You cross."

The following day, the Persians crossed the river, and Mihran arranged his forces in battle order with display of much splendour and pomp.

One wing of the Muslim army was led by Adi b. Hatim, and the other wing was led by Jareer. Masud, a brother of Muthanna held the command of the infantry. Muthanna mounted his horse 'Shams', and rode from one and to the other. Addressing the army he said:

"Brave soldiers! beware, lest, on account of you, the stigma of dishonour should fall to the Arabs."

The Persians dashed forward roaring like thunder. Muthanna shouted to his men not to pay any heed to such noise, as it was mere sound signifying nothing. He asked the wing commanders to stick fast, as he was going to make a rush on the Persian forces.

With the shouts of Allah-o-Akbar the Muslim army rolled forward, and such was the overwhelming impetuosity of their onslaught that they rent asunder the serried ranks of the Persian right flank, and penetrated the Persian centre. The Persians reeled before the terrible onset, but they rallied and fought so desperately that the Muslim ranks began to waver.

Seeing some Muslims turn back, Muthanna thundered: "O Muslims' whither are you going. I am here; come to me. Muthanna rallied his forces and ordered a fresh attack. Masud the brother of Muthanna received many wounds, and fell down. That made the Muslims lose heart. Turning to the Muslims, Muthanna said:

"O Muslims, never mind if my brother is killed. Valiants always die like that. See that the standard that you carry is not lowered."

Masud himself while dying cried, "Let not my death make you lose heart; you must forward to your task."

Anas b. Hilal, a Christian commander fighting with the Muslim forces fell fighting heroically. Muthanna took him up in his arms, and laid him alongside his brother Masud. Many Muslim officers of note were killed, but Muthanna wanted his men to persevere. Mihran the Commander-in-Chief of the Persian army fought heroically. Muthanna asked his men to advance, and make Mihran their target. A youthful warrior of the Taghlab tribe rushed forward with great courage and intrepidity, and penetrating the Persian ranks slew Mihran with his sword. The youth proclaimed:

"I am a young men of the Taghlab tribe;

"I have killed Mihran, the Persian Chief."

The death of Mihran turned the tide of the battle. The Persians lost nerve, and fled in disorder. Muthanna at once made a dash for the bridge and captured it. That prevented the Persians from recrossing the river. The Muslims made mincemeat of the Persians. According to the annals, no battle had ever left so many corpses for its sanguinary souvenir as were strewn on the battle-field of Buwaib. For years thereafter the travellers in the region witnessed the grim spectacle of heaps of bones scattered in all directions.

The battle of Buwaib was the reply of the Muslims to the battle of the Bridge. In the battle of the Bridge a greater part of the Muslim army managed to escape; in the battle of Buwaib the entire Persian army was annihilated.

At the conclusion of the battle, Muthanna said:

"I have fought Arabs and Persians. I have fought them in the time of Ignorance and again in the time of Islam. By Allah during the days of Ignorance a hundred Persians were stronger than a thousand Arabs, but to-day a hundred Arabs are stronger than a thousand Persians."

The battle of the Buwaib was fought in April 635.