Surat Al Anfal: (67-68)

{مَا كَانَ لِنَبِيٍّ أَن يَكُونَ لَهُ أَسْرَى حَتَّى يُثْخِنَ فِي الأَرْضِ تُرِيدُونَ عَرَضَ الدُّنْيَا وَاللّهُ يُرِيدُ الآخِرَةَ وَاللّهُ عَزِيزٌ حَكِيمٌ} [الأنفال : 67]

67- It is not proper for a prophet [i.e., the Prophet (peace be upon him)] to take captives from the polytheists until their power becomes weak and the power of Muslims becomes strong so that they can defeat their enemy.

O Muslims, do you want to accept the ransom instead of killing them?! Allah wants that you gain the reward of the Hereafter, support His Religion, and defeat His Enemy. Allah is the Cherishable who grants victory for His Pious Servants over His Enemy and Wise in ordaining the codes of war.

The Ayah was revealed concerning the captives of Badr Battle when the Prophet (peace be upon him) consulted his Companions about captives, so he inclined to the view of accepting the ransom without killing them.

{لَّوْلاَ كِتَابٌ مِّنَ اللّهِ سَبَقَ لَمَسَّكُمْ فِيمَا أَخَذْتُمْ عَذَابٌ عَظِيمٌ} [الأنفال : 68]

68- Had it been written in the Preserved Tablet that Allah shall not torture a people before He explains to them commands and prohibitions, you would have been inflicted by a great torment due to the ransom which you took from the captives.

It was mentioned that the Companions of the Prophet (peace be upon him) refused to take the ransom, therefore the following Ayah was revealed that you may take it.

The general opinion of jurists has been settled that an Imam is given choice concerning captives: He may kill them or take ransom according to the general benefits of Muslims. It was a benefit for Muslim at that time that disbelievers must have been killed, but Allah forgave that action.