Sahih Bukhari Volume 8, Book 74, Hadith Number 273.

Narated By ‘Aisha : A group of Jews came to Allah’s Apostle and said, “As-samu ‘Alaika ” (Death be on you), and I understood it and said to them, “Alaikum AsSamu wa-l-la’na (Death and curse be on you).” Allah’s Apostle said, “Be calm! O ‘Aisha, for Allah loves that one should be kind and lenient in all matters.” I said. “O Allah’s Apostle! Haven’t you heard what they have said?” Allah’s Apostle said, “I have (already) said (to them), ‘Alaikum (upon you).'”