(4) باب النَّهْىِ عَنِ ابْتِدَاءِ، أَهْلِ الْكِتَابِ بِالسَّلاَمِ وَكَيْفَ يَرُدُّ عَلَيْهِمْ
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). (ID)
Reference (ID): Sahih Muslim 2163 b (ID)
In-book reference (ID): Book 39, Hadith 8 (ID)
USC-MSA web (English) reference(deprecated numbering scheme): Book 26, Hadith 5381 (ID)