Sayyiduna Ma'qil bin Yasar (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "The Muslims are one hand against those other than them; their blood is equal to one another's. A believer is not to be killed in retaliation for a disbeliever, nor one under a covenant during the term of his covenant."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17605