Sunan an-Nasa'i 3166
سنن النسائي#3166SahihIt was narrated from Abu Hurairah that the Messenger of Allah (ﷺ) said:"Allah laughs at two men, one of whom killed the other but they both entered Paradise. The first one fought in the cause of Allah and was killed, then Allah accepted the repentance of the one who killed him, and he fought and was martyred
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Abu GhuddahSahih
Al-AlbaniSahih
Zubair Ali ZaiSahih Bukhari
In-book reference: 25:82