What is the ruling on a Muslim killing someone whose Islam was unknown to the killer until after the killing?

Among the types of accidental killing is when a Muslim kills someone he believes is lawful to kill, such as a disbeliever in war, and then it turns out that person was a Muslim. This is an error that does not necessitate qisas (retribution). Scholars have differed whether diya (blood money) and kaffara (expiation) are obligatory, or kaffara only.

As for killing someone one presumes to be a disbeliever, but who is not permissible to kill (such as dhimmis or musta'minun - those granted safe conduct), this does not absolve the perpetrator from sin and worldly punishment.

As for killing within Dar al-Islam (the abode of Islam): the principle here is caution. If one kills someone presumed to be a disbeliever who turns out to be a Muslim, diya is obligatory. There is a difference of opinion regarding the obligation of qisas, because Dar al-Islam is a land where blood is protected, and therefore verification is mandatory.