Muwatta Malik, Book 21, Hadith 28 (USC-MSA) ⓘ

Yahya related to me from Malik from Abu'z-Zinad from al-Araj from Abu Hurayra that the Messenger of Allah, may Allah bless him and grant him peace, said, "Allah laughs at two men. One of them kills the other, but each of them will enter the Garden: one fights in the way of Allah and is killed, then Allah turns to the killer, so he fights (in the way of Allah) and also becomes a martyr."

وَحَدَّثَنِي عَنْ مَالِكٍ، 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 ‏" ‏ ‏.‏