Musannaf Ibn Abi Shayba 18:1205

Abu Bakr narrated to us, he said: Hafs narrated to us, from Khalid, from Abu Qilabah, from Abu al-Ash'ath, from Shaddad ibn Aws, who attributed it (to the Prophet): He said: "Indeed, Allah has prescribed excellence (ihsan) in all things. So when you kill, perform the killing with excellence."