كتاب الإمارة
33  
The Book on Government
(36)
Chapter: One who kills a disbeliever then keeps to the right path
(36)
باب مَنْ قَتَلَ كَافِرًا ثُمَّ سَدَّدَ ‏‏
Sahih Muslim 1891 b

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."
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 ‏"‏ ‏.‏
Reference : Sahih Muslim 1891b
In-book reference : Book 33, Hadith 192
USC-MSA web (English) reference : Book 20, Hadith 4662
  (deprecated numbering scheme)