Sahih Muslim Book 26, Hadith Number 5386.

Chapter : Prohibition of saying first as-Salam-u-Alaikum to the people of the book, and how their salutations should be responded.

‘Aisha reported that some Jews came to Allah’s Apostle (may peace be upon him) and they said: Abu’l-Qasim (the Kunya of the Holy Prophet), as-Sam-u-‘Alaikum, whereupon he (the Holy Prophet) said: Wa ‘Alaikum. ‘Aisha reported: In response to these words of theirs, I said: But let there be death upon you and disgrace also, whereupon Allah’s Messenger (may peace be upon him) said: ‘Aisha, do not use harsh words. She said: Did you hear what they said? Thereupon he (the Holy Prophet) said: Did I not respond to them when they said that; I said to them: Wa’Alaikum (let it be upon you).