Sahih Bukhari Volume 9, Book 88, Hadith Number 207.

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)