Abu Hurairah reported,
The Messenger of Allah 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 killed, then Allah accepts the repentance of his killer, who later fights and is martyred."In another wording, "Our Lord smiled at two men, one of whom killed the other, yet both are in Paradise."
Azami 371
Sahihعن أبي هريرة
أن رسول الله ﷺ قال: يضحك الله إلى رجلين يقتل أحدهما الآخر، كلاهما يدخل الجنة، يقاتل هذا في سبيل الله فيقتل، ثم يتوب الله على القاتل، فيقاتل فيستشهد. وفي لفظ: ضحك ربنا من رجلين قتل أحدهما صاحبه، وكلاهما في الجنة.
Malik narrated it in Jihad (28) from Abu Zinad, from Aaraj, from Abu Hurairah, and he related it.Bukhari narrated it in Jihad (2826) from Abdullah bin Yusuf, from Malik, with his chain.Muslim narrated it in Imarah (1890) through another route, from Sufyan, from Abu Zinad, with his chain, similarly. He also narrated it through another route from Hammam bin Munabbih, and it is found in his Sahifa (111).The second narration was reported by Ibn Khuzayma in Kitab Tawhid (456), and Ibn Hibban in his Sahih (4666), both from the hadith of Muammal bin Ismail, who said, Sufyan narrated to us, from Abu Zinad, from Aaraj, from Abu Hurairah, and he related the same.Muammal bin Ismail has poor memory, though he was corroborated by others.A similar report was narrated from Anas bin Malik. Ibn Khuzayma reported it in Kitab Tawhid (460) through the route of Bishr bin Husayn Abu Muhammad Asbahani, who said, Zubayr bin Adi narrated to us, from Anas, and he related the hadith similarly.This Bishr is extremely daif. Indeed, Daraqutni said he is matruk, and Abu Hatim said he lies about Zubayr. Dhahabi gave his biography in al-Mizan (1/315).