It is narrated from Sayyiduna Ali (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "The lives of Muslims are equal among themselves; if even the lowest of them grants protection to someone, that protection must be respected. And Muslims are like a single hand against others. Beware! No Muslim should be killed in retaliation for a disbeliever, nor should any dhimmi be killed as long as he is within the period of his covenant and remains upon its conditions."
Hadith Referenceمسند احمد / مسند الخلفاء الراشدين / 991
Hadith Gradingحکم دارالسلام:صحيح لغيره، أبو حسان الأعرج روايته عن على مرسلة