Musnad Ahmad 262:1

Sa'id ibn Mansur narrated to us, al-Mughirah ibn 'Abd al-Rahman narrated to us, from Abu al-Zinad, he said: Muhammad ibn Hamzah al-Aslami narrated to me, from his father: That the Messenger of Allah (peace and blessings of Allah be upon him) appointed him over a military expedition, so I went out in it. He said: "If you capture so-and-so, then burn him with fire." When I turned away, he called me and said: "If you capture him, then kill him, for no one punishes with fire except the Lord of the Fire."