Sayyiduna Samurah bin Jundub (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "Kill the old men of the idolaters and spare their children."
Imam al-Shafi'i (may Allah have mercy on him) says: If the killing of anyone other than monks were to be faulted, then the captive and the wounded would never be killed. Yet in the presence of the Prophet (peace and blessings be upon him), Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) finished off the wounded Abu Jahl bin Hisham.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18164