The Prophet ﷺ said, "Allah smiles upon two men, one of whom kills the other, yet both enter Paradise. One fights in the cause of Allah and is martyred. Then Allah turns in mercy to the killer, who embraces Islam and fights in the cause of Allah until he too is martyred."
حدثنا وكيع قال حدثنا سفيان عن أبي الزناد عن الأعرج
عن النبي ﷺ قال يضحك الله إلى رجلين يقتل أحدهما الآخر كلاهما يدخل الجنة يقاتل هذا في سبيل الله فيستشهد قال ثم يتوب الله على قاتله فيسلم فيقاتل في سبيل الله حتى يستشهد
Its isnad is sahih. Bukhari related it in 4/29, in the Book of Jihad, chapter on a disbeliever who kills a Muslim and then becomes Muslim. Muslim related it in 3/1504, number 1890, in the Book of Leadership. Nasai related it in 6/38, number 3165, in the Book of Jihad, chapter on the killer and the killed meeting together in Paradise. And Malik related it in 1/460, in the Book of Jihad, chapter on martyrs in the path of Allah.9935m, and with its chain of transmission, he said, the Messenger of Allah said, "Do not call the grapevine al-karm, for the true karm is the Muslim man."Sahih (9935m). Its isnad is sahih, like the one before it. Bukhari related it in 8/51, in the Book of Manners, chapter on the statement of the Prophet, "The true karm is the heart of the believer." Muslim related it in 4/1763, number 2247, in the Book of Expressions, chapter on the dislike of calling the grapevine karm. And Abu Dawud, 4974.