Does Qasamah apply when the victim is a free, adult Muslim, regardless of the accused's status?

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When the victim is a free, adult Muslim, there is no difference of opinion; Qasamah applies whether the accused is a Muslim or a disbeliever. The basis for Qasamah is the incident of Abdullah ibn Salhl who was killed in Khaybar, and the Jews were suspected, prompting the Prophet peace be upon him to order Qasamah.