Sahih Muslim Book 20, Hadith Number 4658.

Chapter : The two men, both of whom will enter paradise though one of them slays the other.

It has been narrated on the authority of Abu Huraira that the Messenger of Allah (may peace be upon him) said: God laughs at the two men both of whom will enter Paradise (though) one of them kills the other. They said: Messenger of Allah, how is it? He said: One of them fights in the way of Allah, the Almighty and Exalted and dies a martyr. Then God turns in mercy to the murderer who embraces Islam, fights in the way of Allah, the Almighty and Exalted, and dies a martyr.