Sahih Muslim Book 026, Hadith Number 5381.

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

Anas reported that the Companions of Allah's Apostle (may peace be upon him) said to him: The people of the Book offer us salutations (by saying as-Salamu-'Alaikum). How should we reciprocate them? Thereupon he said: Say: Wa 'Alaikum (and upon you too).