Musnad Ahmad 1018:49

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from Abu ‘Imran al-Jawni, who said: I said to Jundub, "I have pledged allegiance to these people"—meaning Ibn al-Zubayr—"and they want me to go out with them to al-Sham." He said, "Refrain." I said, "They refuse." He said, "Ransom yourself with your wealth." I said, "They refuse unless I fight with them with the sword." Jundub said: So-and-so narrated to me that the Messenger of Allah (peace and blessings of Allah be upon him) said: "The slain person will come with his killer on the Day of Resurrection, and he will say: 'O Lord, ask this one why he killed me.'" Shu‘bah said: And I think he said: "He will say: 'For what did you kill him?' He will say: 'I killed him for the sake of so-and-so’s kingdom.'" Jundub then said: "So beware of it."