Who are the two believers, one of whom killed the other, and both entered Paradise?

The authentic Hadith refers to two men who enter Paradise despite one having fought the other: one fights in the way of Allah and is killed, then Allah forgives his killer, who then attains martyrdom. As for each of them killing the other, this is only known to have occurred among the Companions, whom Allah was pleased with and who entered Paradise. Imam An-Nawawi clarified that the blood of the Companions is not included in the warning of the Hadith, "If two Muslims meet with their swords, then both the killer and the killed are in the Fire." The doctrine of the Ahlus-Sunnah is to think well of them.