Sahih Bukhari Volume 9, Book 92, Hadith 7087

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}‏

Narrated By 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 Apostle (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)