Sunan Ibn Majah, Book 1, Hadith 196 (In-Book) ⓘ

It was narrated that Abu Hurairah said: "The Messenger of Allah said: 'Allah will laugh at two persons- one of them kills the other, and both of them enter Paradise, for the first one fought in the cause of Allah and was martyred, then his killer repented to Allah and became Muslim, then he also fought in the cause of Allah and was martyred.'"

حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا وَكِيعٌ، عَنْ سُفْيَانَ، عَنْ أَبِي الزِّنَادِ، عَنِ الأَعْرَجِ، عَنْ أَبِي هُرَيْرَةَ، قَالَ قَالَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ ‏ "‏ إِنَّ اللَّهَ يَضْحَكُ إِلَى رَجُلَيْنِ يَقْتُلُ أَحَدُهُمَا الآخَرَ كِلاَهُمَا دَخَلَ الْجَنَّةَ يُقَاتِلُ هَذَا فِي سَبِيلِ اللَّهِ فَيُسْتَشْهَدُ ثُمَّ يَتُوبُ اللَّهُ عَلَى قَاتِلِهِ فَيُسْلِمُ فَيُقَاتِلُ فِي سَبِيلِ اللَّهِ فَيُسْتَشْهَدُ ‏" ‏ ‏.‏