From Sahih Muslim (Hadith 5653):
Anas reported that the Companions of Allah's Apostle (ﷺ) 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)
Reference: Muslim 5653