Sahih Bukhari Volume 008, Book 075, Hadith Number 404.

Narated By 'Aisha : The Jews used to greet the Prophet by saying, "As-Samu 'Alaika (i.e., death be upon you), so I understood what they said, and I said to them, "As-Samu 'alaikum wal-la'na (i.e. Death and Allah's Curse be upon you)." The Prophet said, "Be gentle and calm, O 'Aisha, as Allah likes gentleness in all affairs." I said, "O Allah's Prophet! Didn't you hear what they said?" He said, "Didn't you hear me answering them back by saying, 'Alaikum (i.e., the same be upon you)?"