It is narrated from Sayyiduna Ka'b bin Malik (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "When you conquer Egypt, enjoin good treatment of the Copts, for they have a right of protection (dhimmah) and ties of kinship."
Hadith Referenceالمعجم الكبير للطبراني / باب الكاف / 16163