After the defeat of the Muslims at the battle of Uhud, the Quraysh retreating, some of the enemy talked among themselves to attack the Muslims again, since they were weak and wounded. The enemy talking with each other, one group prevailed over other and agreed that they will go back and slaughter more Muslims.
News reached the Prophet (p) that the Quraysh were on their way back to kill more Muslims.
Hence, the companions of Prophet Muhammed (p) which survived in the Battle of Uhud, were to engage the enemy again to protect the Muslim community.
Note, the reports vary on this incident, when it took place precisely, some say the same day, others state the next day after the battle of Uhud.
“…Regarding the Holy Verse: “Those who responded (To the call) of Allah And the Apostle (Muhammad), After being wounded, For those of them Who did good deeds And refrained from wrong, there is a great reward.” (3.172) She said to ‘Urwa, “O my nephew! Your father, Az-Zubair and Abu Bakr were amongst them (i.e. those who responded to the call of Allah and the Apostle on the day (of the battle of Uhud). When Allah’s Apostle, suffered what he suffered on the day of Uhud and the pagans left, the Prophet was afraid that they might return. So he said, ‘Who will go on their (i.e. pagans’) track?’ He then selected seventy men from amongst them (for this purpose).” (Sahih al-Bukhari Volume 5, Book 59, Number 404)
“… Then he left the Messenger of God at Hamra al-Asad and went to Abu Sufyan b. Harb and his men at al-Rawha. They had decided to return to the attack against the Messenger of God and his companions. They argued, ‘We killed a proportion of Muhammad’s companions, including leaders and nobles, and now we have turned back home before exterminating them. Let us return to Medina for the rest of them and finish them off.’ …” 
Kitab Al-tabaqat Al-Kabir – Ibn Sa’d:
“GHAZWAH OF THE APOSTLE OF ALLAH AT HAMRA AL-ASAD
Then (occurred) the ghazwah of the Apostle of Allah against al-Hamra al-Asad on Sunday, 8 Shawwal, after the commencement of the thirty-second month from Hijrah. They said: When the Apostle of Allah returned from Uhud on the evening of Saturday, a group of the leading Ansar passed the night at his door (keeping watch) and the Muslims passed the night in dressing their wounds. When the Apostle of Allah offered morning prayers on Sunday, he asked Bilal to proclaim (or call) that the Apostle of Allah, had ordered them to search their enemy, and that none who had not taken part in the fighting of the preceding days, should come out, Jabir Ibn Abd Allah said (to the Prophet): Verily, my father had left me behind on the day of Uhud to look after my sisters and I had not taken part in the battle, so allow me to march with you. The Apostle of Allah permitted him. Thus none of those who had not participated in the Battle of Uhud, marched with him (Prophet) except him (Jabir). The Apostle of Allah asked for his flag which was folded and not unfurled. He handed it over to Ali Ibn Abi Talib and it has aso been said, to Abu Bakr. … The inhabitants of al-Awali thronged to join him when they heard the cry. The Apostle of Allah rode his horse and the people marched with him. He despatched three persons of the Banu Aslams as vanguard who were to follow the foot-steps of the (polytheists) people. Two of them met (the polytheists) at Hamra al-Asad. It is ten miles from al-Madinah, on the way of al-Aqiq to the left of Dhu al-Hulayfah, by the route of the valley. The people (polytheists) were noisy and counselling to return but Safwan Ibn Umayyah was stopping them. They noticed these two men, overpowered them, killed them and prococeeded on. The Apostle of Allah reached there with his companions and encamped at Hamra al-Asad. He interred the two men in one grave; they were relatives. …” 
Bashir Aḥmad M.A.:
“Ghazwah of ḤamrA’ul-Asad
This night was a night of great fear in Madinah, because although the army of the Quraish had apparently taken to Makkah, it was apprehended that this move may be a plot to catch the Muslims off guard, and suddenly return to attack Madinah. Hence, on this night an arrangement was made for security in Madinah and the Companions particularly stood guard all night long at the residence of the Holy Prophet. The next morning it was discovered that this apprehension was not mere speculation, because prior to Fajr Ṣalat, the Holy Prophet received news that the army of the Quraish had stalled at a few miles from Madinah and a heated debate was taking place amongst the chieftains of
Makkah, that making use of this victory, why not attack Madinah. Some of the Quraish were taunting one another saying:
“You did not kill Muḥammad [sa], nor did you take the Muslim women as slaves, nor did you seize their wealth and possessions; rather, when you gained dominance over them and received the opportunity to destroy them completely, you just left them and turned back, so that they may collect strength again.
There is still time, let us return and attack Madinah and uproot the Muslims once and for all.”
In contrast to this, the others argued:
“You have attained victory. Consider this to be good fortune enough and return to Makkah, lest we lose this repute as well, and this victory is turned to defeat. Now if you return and attack Madinah, the Muslims shall indeed fight with all their strength, and those who did not participate at Uhud shall come forth in the field of battle as well.”2 Ultimately, however, the opinion of the passionate ones prevailed and the Quraish prepared to set back to Madinah. When the Holy Prophet was informed of these events, he immediately announced that the Muslims should prepare, but along with this he also ordered that except for those people who had participated at Uhud, no one else should set out with them.
As such, the warriors of Uhud, most of whom were wounded, tied up their wounds and betook to the company of their Master. It is written that on this occasion, the Muslims set out with such joy and zeal, as if they were a triumphant army which sets out in pursuit of the enemy. After travelling a distance of eight miles, the Holy Prophet reached Ḥamra’ul-Asad, where the dead bodies of two Muslims were found laying on the plain. Upon further investigation, it was discovered that these were the two spies whom the Holy Prophet had sent in the footsteps of the Quraish. Upon finding an opportunity, the Quraish had executed them. The Holy Prophet instructed that a single grave be dug, and both of them were buried together. Now that it was evening time, the Holy Prophet instructed that camp be set up at this very location. He further instructed that fires be lit at different places throughout the plain. As such, in no time, 500 fires were lit in the plain of Ḥamra’ul-Asad, which struck awe into the heart of any spectator from afar. Most probably, on this occasion, an idolatrous chief of the Khuza‘ah tribe named Ma‘bad presented himself before the Holy Prophet and offered his condolences for those who had fallen at Uhud, where after he continued on his way. On the following day when he reached Rauha’, lo and behold, he found the army of the Quraish encamped there and that preparations were under-way to return to Madinah. Ma‘bad went to Abū Sufyan at once and said:
“What are you about to do? By God, I have just let behind the army of Muḥammad [sa] at Ḥamra’ul-Asad, and never before have I seen such an awe-inspiring army. They are so passionate due to their regret for the defeat at Uhud that they shall reduce you to ashes on sight.” Abu Sufyan and his followers were so awe-struck by these comments of Ma‘bad, that they abandoned the idea of returning to Madinah and made haste to Makkah at once. When the Holy Prophet received news that the army of the Quraish had fled in this manner, he thanked God and said, “This is the awe of God, which He has struck into the hearts of the disbelievers.”2 After this, the Holy Prophet remained in Ḥamra’ul-Asad for another two or three days, and after an absence of five days, he returned to Madinah. In this campaign, two warriors from among the Quraish, one of whom was treacherous and the other a spy, were captured by the Muslims. According to the laws of war, since their punishment was death, they were executed by order of the Holy Prophet. From among them, one was a renowned poet of Makkah named Abu ‘Uzzah, who was taken captive at the hands of the Muslims in the battle of Badr. On that occasion, the Holy Prophet released him without ransom upon his seeking forgiveness and promising that he would never come forth to fight the Muslims again. However, he betrayed the Muslims and participated in war against the Muslims again. Moreover, not only did he participate himself, but with his provocative couplets, he incited others as well. Hence, since the treachery of such a man could have proven to be severely injurious to the Muslims, when he was now taken captive at the hands of the Muslims a second time, the Holy Prophet ordered that he be put to death. Abu ‘Uzzah attempted to talk his way to freedom again with a verbal apology, but the Holy Prophet refused and said:
“A believer is not stung by the same hold twice.” The second captive was Mu‘awiyyah bin Mughirah. This individual was from among the relatives of Ḥadrat ‘Uthmān bin ‘Affan, but was a staunch enemy of Islam. After the battle of Uhud, he continued to secretly circle the vicinity of Madinah, but the Companions spotted him, apprehended him and presented him before the Holy Prophet. Ḥaḍrat ‘Uthman interceded on his behalf and the Holy Prophet released him on the condition that he would leave within three days, otherwise he would be executed according to the punishment which was meted out to spies. Mu‘awiyyah promised that he would leave within three days, but when this time limit had expired and he was still found to be circling Madinah secretly just as before, he was executed.1 History has not recorded his motive, but for someone to secretly remain in the vicinity of Madinah, and then to remain there past the stipulated term despite being warned, demonstrates that he harboured dangerous intent. It is very likely that agitated and vexed by the safe return of the Holy Prophet from the field of Uhud, he came to Madinah with some ill-motive against the very person of the Holy Prophet and planned to make a secret strike with the help of a conspiracy hatched by the Jews or idolators of Madinah. Nonetheless, God the Exalted granted His protection and his plan could not succeed.” 
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 The History of al-Tabari: The Foundation of the Community: Muhammad, volume 7, page 140
 Kitab Al-tabaqat Al-Kabir, Ibn Sa’d – volume 2, page 57 – 59
 The Life & Character of the Seal of Prophets – (Sirat Khatamun-Nabiyyin) By Mirza Bashir Aḥmad M.A., volume 2, page 350 – 353