From Sahih Muslim (Hadith 4896):
It has been narrated on the authority of Abu Huraira that the Messenger of Allah (ﷺ) 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
Reference: Muslim 4896