Ma'qil bin Yasar (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "A believer will not be killed in return for a disbeliever, nor a dhimmi during his covenant; the Muslims are to be helped against those besides them, and their blood is equal."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15916