It is narrated from Sayyiduna Anas (may Allah be pleased with him), he says: A man from among the People of the Book came and greeted the Noble Prophet (peace and blessings be upon him) saying: "As-Saam ‘alaykum" (death be upon you). Sayyiduna ‘Umar (may Allah be pleased with him) said: O Messenger of Allah! Shall I not strike off his neck? The Prophet (peace and blessings be upon him) said: No, just when these people greet you, respond to them by saying: "Wa ‘alaykum" (and upon you).