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."
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ عَوْنٍ الْهِلاَلِيُّ، حَدَّثَنَا أَبُو إِسْحَاقَ الْفَزَارِيُّ، إِبْرَاهِيمُ بْنُ مُحَمَّدٍ عَنْ سُهَيْلِ بْنِ أَبِي صَالِحٍ، عَنْ أَبِيهِ، عَنْ أَبِي هُرَيْرَةَ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " لاَ يَجْتَمِعَانِ فِي النَّارِ اجْتِمَاعًا يَضُرُّ أَحَدُهُمَا الآخَرَ " . قِيلَ مَنْ هُمْ يَا رَسُولَ اللَّهِ قَالَ " مُؤْمِنٌ قَتَلَ كَافِرًا ثُمَّ سَدَّدَ " .
sunnah.com