Sahih Muslim Book 20, Hadith Number 4662.

Chapter : About a man who killed a disbeliever and embraced Islam.

It has been narrated on the authority of Abu Huraira that the Messenger of Allah (may peace be upon him) said: No two such persons shall be together in Hell as if one of them is such that his presence hurts the other. It was asked: Messenger of Allah, who are they? He said: A believer who killed a disbeliever and (then) kept to the right path.”