Yahya related to me from Malik from Abdullah ibn Dinar thatAbdullah ibn Umar said that the Messenger of Allah, may Allah blesshim and grant him peace, said, "When a jew greets you, and says 'Deathto you' (as-samu alaykum) say, 'And to you.' "Yahya said,"Malik was asked whether a person who greeted a jew or christian,should apologise for it. He said, 'No'."
USC-MSA web (English) reference: Book 53, Hadith 3
Arabic reference: Book 53, Hadith 1761