What is the ruling on killing a Muslim in the Abode of War (Dar al-Harb) while mistaking him for an enemy combatant?

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One who kills a Muslim in the Abode of War (Dar al-Harb) while mistakenly believing him to be an unbeliever, or one who shoots toward the enemy ranks and strikes and kills a Muslim among them, is obligated to offer expiation (Kaffarah). This ruling is based on the saying of Allah the Almighty: {But if he was of a people with whom you have a treaty, then [there is due] the freeing of a believing slave} (Quran 4:92).