The battle of Mootah was fought in September 629. In the following month, the Prophet received reports that the tribesmen of Qadha'a were massing in the north of Wadi-ul-Qura with the intent of raiding Medina. This was a direct result of the defeat of the Muslims at Mootah. The pagan tribes believed that the power of the Muslim was broken at Mootah, and that if they attacked Medina, they would hardly meet any resistance.
The Prophet had to take counter-measures to forestall a tribal excursion into Medina. He, therefore, sent three hundred soldiers under the command of Amr bin Aas, to watch the Qadha'a in their own territory, and to disperse them, if necessary.
Amr left Medina, and halted in the north of Wadi-ul-Qura, near a spring called Dhat es-Salasil. He was alarmed to see multitudes of armed tribesmen roving in the valley and sent a messenger to the Prophet requesting reinforcements. The Prophet immediately complied, and sent another two hundred men under the command of Abu Obaidah ibn al-Jarrah.This second group included both Abu Bakr and Umar.
When Abu Obaidah arrived in the camp of Amr bin Aas, he indicated that he would like to take command of both contingents. But Amr's answer to this suggestion was an emphatic no. He made it clear to Abu Obaidah that he (Amr) was the supreme commander of all the troops, his own as well as the reinforcements which the latter had brought, all five hundred men.
At night there was a sudden drop in the temperature, and the weather became unseasonably cold. The troopers lighted small fires for warmth, and sat around them. Amr, however, ordered them to put them out. All of them obeyed except Abu Bakr and Umar. Amr repeated his order. But they still demurred whereupon Amr threatened to throw both of them into it if they persisted in disobeying him. Umar turned to Abu Bakr and complained to him about the brusque and abrupt manner of Amr.Abu Bakr told him that Amr understood the art of war better than they did, and therefore they ought to obey him. They then extinguished the fire.
On the following day there was some desultory fighting but the tribesmen fought without any order or discipline and were soon dispersed. The Muslims wanted to pursue them into the hills and valleys but Amr forbade them to do so. The tribesmen had abandoned their baggage and the Muslims collected it. They also captured many camels and sheep, and then returned to Medina.
During the campaign, and on the return journey, Amr bin Aas led his troops in prayers. He thus demonstrated to them that he was their commander in both spheres – military and religious. Abu Obaidah, Abu Bakr and Umar, all three, took their orders from him, and said their prayers behind him.
When the expedition returned to Medina, Umar complained to the Prophet about the unceremonious and highhanded manner in which his commanding officer, Amr bin Aas, had treated him and Abu Bakr at Dhat es-Salasil. It was a custom of the Prophet to debrief his captains when they returned from an expedition. They had to give him a comprehensive report on the conduct of the campaign.
Amr was ready to defend his actions. He told the Prophet that the Muslims were very few, and the bonfires would have betrayed their lack of numbers to the enemy. It was in the interests of their own security, he said, that he had ordered them to extinguish them. He further said that the reason why he forbade his men to pursue the enemy was that the latter was in his own territory, and could have easily regrouped to attack them. The Muslims, he pointed out, were fighting in unfamiliar country, and were, therefore, at a disadvantage. The Prophet was satisfied with Amr's explanation, and dismissed Umar's complaints.
Sir William Muir
The repulse of his army from Mootah affected dangerously the prestige of Mohammed among the tribes of the Syrian frontier. There were rumors that the Bedouin tribes of that neighborhood had assembled in great force, and were even threatening a descent upon Medina. Amru, the new convert, was therefore placed at the head of three hundred men including thirty horse, with instructions to subjugate the hostile tribes and incite those whom he found friendly, to harass the Syrian border.
After a march of ten days he encamped at a spring near the Syrian confines. There he found that the enemy was assembled in great numbers, and that he could look for little aid from the local tribes. He halted and dispatched a messenger for reinforcements. Mohammed at once complied, and sent two hundred men, among whom were both Abu Bakr and Omar, under the command of Abu Obeidah. On joining Amr, Abu Obeidah wished to assume the leadership of the whole force, or at the least to retain the chief authority over his own detachment; but Amru, giving promise of the decision and firmness which characterized him in after days, insisted on retaining the sole command. Abu Obeidah, a man of mild and pliant temper, succumbed. "If you refusest to acknowledge my authority," he said, "I have no resource but to obey thee; for the Prophet strictly charged me to suffer no altercation, nor any division of command." Amru replied imperiously: "I am the chief over thee. Thou has only brought a reinforcement to my army." "Be it so," said Abu Obeidah. Amru then assumed command of the united troops, and led their prayers; for thus early were the spiritual functions in Islam blended with the political and military. (The Life of Mohammed, London, 1877)
Muhammad Husayn Haykal
A few weeks after the return of Khalid, Muhammad sought to make up the losses in Muslim prestige in the northern parts of the peninsula which the previous engagement with the Byzantines had caused. He, therefore, commissioned Amr ibn al-Aas to rouse the Arabs to march against al-Sham. He chose Amr for this task because the latter's mother belonged to one of the northern tribes, and he hoped that Amr could use this connection to facilitate his mission. As he arrived at a well called al-Salassil, in the land of Judham, fearing the enemy might overtake him, he sent word to the Prophet asking for more forces. The Prophet sent Abu Ubaydah ibn al Jarrah at the head of a corps of Muhajirun which included Abu Bakr and Umar... (The Life of Muhammad, Cairo, 1935)
Amr bin Aas was a new Muslim. But once he became a Muslim, he rose very rapidly from ranker to general in the army of Medina. He was, it is obvious, endowed with extraordinary ability both as a general and an administrator. The Prophet, therefore, placed men who were many years older than him, and who had accepted Islam long before him, under his command.
Abu Obaidah and Abu Bakr had become Muslim twenty years before Amr, and thus represented the "brass" in Islam whereas Amr bin Aas was only a "rookie" in faith at this time. And yet the Prophet ordered Abu Obaidah to serve under Amr.
This only proves that when the time came for the Prophet to select a man to take command in a certain situation, he took into account, not his age, but his ability – the ability to get results!