Musnad Ahmad 28:281

Abu Kamil narrated to us, Hammad—meaning Ibn Salamah—narrated to us, Habib al-Mu'allim informed me, from 'Amr ibn Shu'ayb, from his father, from his grandfather, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, the most defiant of people against Allah, the Almighty and Majestic, is he who kills within the Sacred Precinct of Allah, or kills someone other than his killer, or kills for the blood feuds of the Pre-Islamic Era (Jahiliyyah)."