It is narrated from Sayyiduna Samurah bin Jundub (may Allah be pleased with him); he said: The Messenger of Allah (peace and blessings be upon him) said: "None of you should wrongfully kill his companion's captive, killing him as soon as he seizes him."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 7068