Musnad Ahmad 28:266

'Abd al-Samad narrated to us, Muhammad ibn Rashid narrated to us, Sulayman narrated to us, from 'Amr ibn Shu'ayb, from his father, from his grandfather, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever bears arms against us is not one of us, nor is he who lies in wait on the road. Whoever kills [someone] in a manner other than that, it is quasi-intentional, its blood money is aggravated, and its perpetrator is not to be killed; it is like the Sacred Month in terms of sanctity and protection."