Musnad Ahmad 28:186

Husayn ibn Muhammad and Hashim—meaning Ibn al-Qasim—narrated to us, they said: Muhammad ibn Rashid al-Khuza'i narrated to us, from Sulayman ibn Musa, from 'Amr ibn Shu'ayb, from his father, from his grandfather: "That the Prophet, may Allah bless him and grant him peace, decreed that a Muslim shall not be killed for a disbeliever."