Sahih Bukhari Volume 8, Book 73, Hadith Number 53.

Narated By ‘Aisha : (The wife of the Prophet) A group of Jews entered upon the Prophet and said, “As-Samu-Alaikum.” (i.e. death be upon you). I understood it and said, “Wa-Alaikum As-Samu wal-la’n. (death and the curse of Allah be Upon you).” Allah’s Apostle said “Be calm, O ‘Aisha! Allah loves that on, should be kind and lenient in all matters.” I said, “O Allah’s Apostle! Haven’t you heard what they (the Jews) have said?” Allah’s Apostle said “I have (already) said (to them) “And upon you!”