Sahih Muslim Book - 33, Hadith - 4892

حَدَّثَنَا مُحَمَّدُ بْنُ أَبِي عُمَرَ الْمَكِّيُّ ، حَدَّثَنَا سُفْيَانُ ، عَنْ أَبِي الزِّنَادِ ، عَنِ الأَعْرَجِ ، عَنْ أَبِي هُرَيْرَةَ ، أَنَّ رَسُولَ اللهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ : يَضْحَكُ اللَّهُ إِلَى رَجُلَيْنِ ، يَقْتُلُ أَحَدُهُمَا الآخَرَ كِلاَهُمَا يَدْخُلُ الْجَنَّةَ ، فَقَالُوا : كَيْفَ يَا رَسُولَ اللهِ ، ؟ قَالَ : يُقَاتِلُ هَذَا فِي سَبِيلِ اللهِ عَزَّ وَجَلَّ فَيُسْتَشْهَدُ ، ثُمَّ يَتُوبُ اللَّهُ عَلَى الْقَاتِلِ ، فَيُسْلِمُ ، فَيُقَاتِلُ فِي سَبِيلِ اللهِ عَزَّ وَجَلَّ فَيُسْتَشْهَدُ.

It was narrated from Abu Hurairah that the Messenger of Allah (s.a.w) said: "Allah will laugh at two men, one of whom kills the other, and both of whom will enter Paradise." They said: "How is that, O Messenger of Allah?" He said: "One fights in the cause of Allah and is martyred, then Allah turns in forgiveness to the one who killed him as he becomes Muslim, and he fights in the cause of Allah and is martyred."