Musnad Ahmad 28:269

Isma'il ibn Muhammad—meaning Abu Ibrahim al-Mu'aqqib—narrated to us, Marwan narrated to us, al-Hasan ibn 'Amr al-Fuqaymi narrated to us, [from Mujahid], from Junadah ibn Abi Umayyah, from 'Abdullah ibn 'Amr, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever kills a person from the people of the covenant (ahl al-dhimmah) will not smell the fragrance of Paradise, and indeed its fragrance is found from a distance of forty years."