Full Sharh — Mishkat al-Masabih 3646
وَعَنْ أَبِي هُرَيْرَةَ أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ: " يَضْحَكُ اللَّهُ تَعَالَى إِلَى رَجُلَيْنِ يَقْتُلُ أَحَدُهُمَا الْآخَرَ يَدْخُلَانِ الْجَنَّةَ: يُقَاتِلُ هَذَا فِي سَبِيلِ اللَّهِ فَيُقْتَلُ ثُمَّ يَتُوبُ اللَّهُ على الْقَاتِل فيستشهد "
English Translation
Abu Huraira reported God's Messenger as saying, "God most high will cheerfully welcome two men one of whom kills the other. They will enter paradise, one having fought in God’s path and been killed, and the slayer, after receiving God's pardon, having died as a martyr."* * The man who had fought and killed a Muslim later became a Muslim himself and was killed in the course of jihad. (Bukhari and Muslim.)
Source
Mishkat al-Masabih(مشكاة المصابيح)
Jihad
Hadith Number
3646