Riyad as-Salihin 24

Messenger of Allah (sallallahu alayhi wa sallam) said, “Allah, the Exalted, smiles at two men, one of them killed the other and both will enter Jannah. The first is killed by the other while he is fighting in the Cause of Allah, and thereafter Allah will turn in mercy to the second and guide him to accept Islam and then he dies as a Shaheed (martyr) fighting in the Cause of Allah.”