Sahih al-Bukhari 7085
Narrated Abu Al-Aswad:
An army unit was being recruited from the people of Medina and my name was written among them. 
Then I met `Ikrima, and when I informed him about it, he discouraged me very strongly and said, "Ibn 
`Abbas told me that there were some Muslims who were with the pagans to increase their number 
against Allah's Messenger (ﷺ) (and the Muslim army) so arrows (from the Muslim army) would hit one of 
them and kill him or a Muslim would strike him (with his sword) and kill him. So Allah revealed:-- 
'Verily! As for those whom the angels take (in death) while they are wronging themselves (by staying 
among the disbelievers).' (4.97)
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ يَزِيدَ، حَدَّثَنَا حَيْوَةُ، وَغَيْرُهُ، قَالاَ حَدَّثَنَا أَبُو الأَسْوَدِ،‏.‏ وَقَالَ اللَّيْثُ عَنْ أَبِي الأَسْوَدِ، قَالَ قُطِعَ عَلَى أَهْلِ الْمَدِينَةِ بَعْثٌ فَاكْتُتِبْتُ فِيهِ فَلَقِيتُ عِكْرِمَةَ فَأَخْبَرْتُهُ فَنَهَانِي أَشَدَّ النَّهْىِ ثُمَّ قَالَ أَخْبَرَنِي ابْنُ عَبَّاسٍ أَنَّ أُنَاسًا مِنَ الْمُسْلِمِينَ كَانُوا مَعَ الْمُشْرِكِينَ يُكَثِّرُونَ سَوَادَ الْمُشْرِكِينَ عَلَى رَسُولِ اللَّهِ صلى الله عليه وسلم فَيَأْتِي السَّهْمُ فَيُرْمَى فَيُصِيبُ أَحَدَهُمْ، فَيَقْتُلُهُ أَوْ يَضْرِبُهُ فَيَقْتُلُهُ‏.‏ فَأَنْزَلَ اللَّهُ تَعَالَى ‏{‏إِنَّ الَّذِينَ تَوَفَّاهُمُ الْمَلاَئِكَةُ ظَالِمِي أَنْفُسِهِمْ‏}‏