Sahih Muslim | Book 26 | Hadith Number 5381

Narrated by Anas
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).