Musnad Ahmad 28:214

Waki’ narrated to us, Khalifah ibn Khayyat narrated to us, from ‘Amr ibn Shu’ayb, from his father, from his grandfather, that the Prophet, peace and blessings of Allah be upon him, said in his sermon while his back was leaning against the Ka’bah: "A Muslim shall not be killed for a disbeliever, nor shall a person under a covenant be killed while in his covenant."