I said, "Tell me, O Messenger of Allah, if I meet an infidel, and we fight together, and he cuts off my hands with his sword, then hides from me behind a tree and (then) says he has submitted himself to Allah. Shall I kill him after he has said it?" He (ﷺ) replied, "Do not kill him." I submitted, "But O Messenger of Allah, he cut off one of my hands and only then he said it." Messenger of Allah (ﷺ) then replied, "Do not kill him, for if you do so, he will be in the position in which you were before you killed him (i.e., he will be considered a Muslim and thus his life will be inviolable), and you will be in the position in which he was before he made his testimony (i.e., your life will not be inviolable, for his heirs can ask for Qisas)".

[Al-Bukhari and Muslim].