Surah Muhammad 47:4 محمد

O believers, when you come face-to-face with those disbelievers who wage war against you, strike their necks with your swords and persevere in fighting them until you subdue them and eradicate their power. Once you have stopped them, tie the chains of the captives. When you capture them, as dictated by common interests [of the Muslims], you can be graceful to them by releasing them for no ransom, ransoming them for wealth or something else, killing them, or enslaving them. Continue fighting and capturing them until the war ends by the disbelievers accepting Islam or by your entering into a treaty with them. The mentioned trial of the believers with the disbelievers, the alternation of fortunes, and the victory of one over the other is Allah's decision. If Allah willed to defeat the disbelievers without a fight, He would have done so, but He ordained striving in His path to test some of you with others. He pushes to see who of the believers fights and who doesn’t. He tries the disbeliever with the believer: If the believer is killed, he enters Paradise; if the believer kills him, he enters the Fire. Those who are killed in Allah's path, Allah will never let their deeds go to waste.