Musnad Ahmad 799:68

Abu Mu'awiyah narrated to us, Hajjaj narrated to us, from Qatadah, from al-Hasan, from Samurah ibn Jundub, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Kill the old men of the polytheists, and spare their youths." Abdullah said: I asked my father about the interpretation of this hadith: "Kill the old men of the polytheists." He said: He means: "The old man is unlikely to accept Islam, and the youth, meaning he accepts Islam, as if he is closer to Islam than the old man." He said: "The *sharkh* (youths): the young men."