﴿ يَٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوٓا۟ إِذَا ضَرَبْتُمْ فِى سَبِيلِ ٱللَّهِ فَتَبَيَّنُوا۟ وَلَا تَقُولُوا۟ لِمَنْ أَلْقَىٰٓ إِلَيْكُمُ ٱلسَّلَٰمَ لَسْتَ مُؤْمِنًۭا تَبْتَغُونَ عَرَضَ ٱلْحَيَوٰةِ ٱلدُّنْيَا فَعِندَ ٱللَّهِ مَغَانِمُ كَثِيرَةٌۭ ۚ كَذَٰلِكَ كُنتُم مِّن قَبْلُ فَمَنَّ ٱللَّهُ عَلَيْكُمْ فَتَبَيَّنُوٓا۟ ۚ إِنَّ ٱللَّهَ كَانَ بِمَا تَعْمَلُونَ خَبِيرًۭا ﴾

“[Hence,] O you who have attained to faith, when you go forth [to war] in God's cause, use your discernment, and do not - out of a desire for the fleeting gains of this worldly life - say unto anyone who offers you the greeting of peace, "Thou art not a believer" for with God there are gains abundant. You, too, were once in the same condition - but God has been gracious unto you. Use, therefore, your discernment: verily, God is always aware of what you do.”

Then Allah revealed about Usamah Ibn Zayd who killed Mirdas Ibn Nuhayk al-Farari, a fellow believer, saying: (O ye who believe! When ye go forth (to fight) in the way of Allah) in jihad, (be careful to discriminate) between who is a believer and who is an unbeliever, (and say not unto one who offereth you peace) he who greets you and says to you �there is no god but Allah, Muhammad is the Messenger of Allah�: (�Thou art not a believer�;) and then kill him (seeking the chance profits of this life) so that you may despoil him. (With Allah are plenteous spoils) He has with Him a tremendous reward for him who avoids killing a fellow believer. (Even thus (as he now is) were ye before) with your people feeling safe with the believers and with Muhammad (pbuh) and his Companions because of the attestation of �there is no god but Allah and Muhammad is the Messenger of Allah�, before the Emigration; (but Allah hath since then been gracious unto you) by migrating from the unbelievers. (Therefore take care to discriminate) such that you do not kill believers. (Allah is ever informed of what ye do) whether it is killing someone or other things.