Greeting and the Reply
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).
Sahih Muslim 2163b