Musannaf Ibn Abi Shayba 18:748

Abu Bakr narrated to us, he said: Ibn Abi Ishaq narrated to us, from Muhammad ibn Ishaq, from ‘Amr ibn Shu‘ayb, from his father, from his grandfather, from the Prophet, may Allah bless him and grant him peace, who said: "A believer shall not be killed for a disbeliever."