Musnad Ahmad 529:21

Ya'mar ibn Bishr narrated to us, Abdullah narrated to us, Safwan ibn 'Amr informed us, that Abu al-Muthanna al-Mulayki narrated to him that he heard 'Utbah ibn 'Abd al-Sulami—and he was one of the companions of the Prophet, may God bless him and grant him peace—narrating from the Messenger of God, may God bless him and grant him peace, who said: "Killing is of three types." He then mentioned its meaning.