Sayyidah Aishah (may Allah be pleased with her) says: A group of Jews came to the Messenger of Allah (peace and blessings be upon him) and said: "As-samu 'alaykum" (death be upon you). Sayyidah Aishah (may Allah be pleased with her) says: I understood what they said and replied: "Wa 'alaykumus-samu wal-la'nah" (and upon you be death and curse). Sayyidah Aishah (may Allah be pleased with her) says that the Prophet (peace and blessings be upon him) said: "O Aishah (may Allah be pleased with her)! Be gentle, for Allah, the Lord of Might, loves gentleness in all matters." She said: "O Messenger of Allah (peace and blessings be upon him)! Did you not hear what they said?" He (peace and blessings be upon him) said: "I did reply: 'Wa 'alaykum' (and upon you be death)."
Note: It is necessary that there be some distinguishing mark by which they can be told apart from the Muslims.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجزية / 18723