Sahih Muslim
The Book on Salutations and Greetings (Kitab As-Salam)
Book Number: 026 / Hadith Number: 5381
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).