Understand this hadith ＋
When it was the battle of Badr, the Messenger of Allah (may Allah's peace and blessings be upon him) looked at the polytheists, who were numbered one thousand fighters, whereas his Companions consisted of only three hundred and nineteen men. As the Muslims were a minority compared to the disbelievers, the Prophet (may Allah's peace and blessings be upon him) turned toward the Qiblah and raised his hands to the heaven, audibly invoking Allah: "O Allah, fulfill for me what You have promised to me. O Allah, accomplish what You have promised to me. O Allah, if this small band of Muslims is destroyed, You will not be worshiped on earth." He continued his prayer until his cloak fell off his shoulders. Abu Bakr picked it up and put it back on the Prophet's shoulders, embracing him from behind and telling him that he made sufficient invocation to Allah, Who would fulfill His promise of victory to the Prophet. Hence, Allah, the Almighty, revealed: {When you appealed to your Lord for help, He responded to your call (saying): I will help you with one thousand angels coming in succession} [Sūrat al-Anfāl: 9]. This means that Allah would provide the believers with a thousand fighters among the angels who would be sent one after another to support the believers in their fight against the polytheists.
Ibn ‘Abbās mentioned that while a Muslim man from among the Ansār was chasing a disbeliever to kill him, he heard a crack of a whip and the voice of a horseman saying "Go ahead, Hayzūm." The Ansāri looked to find the infidel on the ground with the trace of a whip on his nose and a rupture across his face. The Ansāri told the Prophet (may Allah's peace and blessings be upon him) about that, and the Prophet told him that this was an angel from the third heaven and that Hayzūm was the name of that angel's horse. On that day, the Muslims killed seventy disbelievers and took another seventy as captives.
Concerning the captives, the Prophet (may Allah's peace and blessings be upon him) consulted Abu Bakr and ‘Umar on what to do with them. Abu Bakr opined that those were the Muslims' relatives and kin, and therefore the Prophet should take a financial ransom and release them. The ransom would support the Muslims in their subsequent fights against the disbelievers. Abu Bakr added that, perhaps, Allah would guide them to Islam. ‘Umar, however, did not agree with Abu Bakr's opinion and opined that the Muslims should kill them and the Prophet should ask each Muslim to kill his relative among these captives, for they were the leaders of disbelief and the heads of misguidance. The Prophet (may Allah's peace and blessings be upon him) inclined to Abu Bakr's suggestion rather than that of ‘Umar.
One day later, ‘Umar found the Prophet and Abu Bakr crying. He asked why they were crying, for if there was a reason for crying, he would join them in it, and if not, then he would pretend to cry in sympathy with them. The Messenger (may Allah's peace and blessings be upon him) told him that they were crying because Allah spoke of the punishment of those who spoke of accepting the ransom from the captive disbelievers; the Prophet (may Allah's peace and blessings be upon him) pointed to a tree near him and said that the punishment was presented closer than that tree. In this regard, Allah, the Almighty, revealed: {It is not befitting for a Prophet that he should take prisoners until the force of the disbelievers has been crushed...} [Sūrat al-Anfāl: 67] to the end of the verse: {So eat of what you have gained of the spoils of war, (it is) lawful and pure} [Sūrat al-Anfāl: 69]. This means that it is not appropriate for a Prophet, while fighting the disbelievers who want to extinguish the light of Allah and extirpate Islam, to hasten to arrest them and keep them alive to collect their ransom. Such ransom is of little value compared with the benefit of eradicating them and ending their evil. As long as those infidels were strong enough to act maliciously and threaten the Muslims, it was not appropriate to take them as captives. However, if they were incapable of doing evil, it would be fine to arrest them and keep them alive. Then Allah declared the riches that the Muslims take from the disbelievers by force in war to be lawful for them.
