Chapter 37: The Book of Fighting [The Prohibition of Bloodshed]
It was narrated that Abu 'Imran Al-Jawni said: "Jundab said: 'So and so told me that the Messenger of Allah [SAW] said: The slain will bring his killer on the Day of Resurrection and will say: Ask him why he killed me. He will say: I killed him defending the kingdom of so and so.'" Jundab said: "So be careful.'"
Sunan an-Nasa'i 3998