Sahih Muslim Book 20, Hadith Number 4660.

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

It has been reported on the authority of Abu Huraira that the Messenger of Allah (may peace be upon him) said: God laughs at the two men one of whom kills the other; both of them will enter Paradise. They (the Companions) said: How, Messenger of Allah? He said: One is slain (in the way of Allah) and enters Paradise. Then God forgives the other and guides him to Islam; then he fights in the way of Allah and dies a martyr.