Sunan an-Nasa'i, Book 25, Hadith 82 (In-Book) ⓘ

It was narrated from Abu Hurairah that the Messenger of Allah said: "Allah laughs at two men, one of whom killed the other but they both entered Paradise. The first one fought in the cause of Allah and was killed, then Allah accepted the repentance of the one who killed him, and he fought and was martyred."

أَخْبَرَنَا مُحَمَّدُ بْنُ سَلَمَةَ، وَالْحَارِثُ بْنُ مِسْكِينٍ، قِرَاءَةً عَلَيْهِ وَأَنَا أَسْمَعُ، عَنِ ابْنِ الْقَاسِمِ، قَالَ حَدَّثَنِي مَالِكٌ، عَنْ أَبِي الزِّنَادِ، عَنِ الأَعْرَجِ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ يَضْحَكُ اللَّهُ إِلَى رَجُلَيْنِ يَقْتُلُ أَحَدُهُمَا الآخَرَ كِلاَهُمَا يَدْخُلُ الْجَنَّةَ يُقَاتِلُ هَذَا فِي سَبِيلِ اللَّهِ فَيُقْتَلُ ثُمَّ يَتُوبُ اللَّهُ عَلَى الْقَاتِلِ فَيُقَاتِلُ فَيُسْتَشْهَدُ ‏" ‏ ‏.‏