The Messenger of Allah ﷺ said, "None of you should take his brother's captive and kill him."
حدثنا يزيد أخبرنا بقية بن الوليد عن إسحاق بن ثعلبة عن مكحول
قال رسول الله ﷺ لا يتعاطى أحدكم من أسير أخيه فيقتله
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Its chain of transmission is daif due to Ishaq bin Thalaba Himyari Himsi, Abu Safwan, whom they declared daif because he brings strange narrations that no one else transmits, and this is one of them. Haythami likewise declared it daif in 5/333 for this same reason.