The Companions of the prophet ﷺ said to the prophet ﷺ: The people of the Book salute us. How should we reply to them? He said: say : the same to you. (Using translation from Abū Dāʾūd 5207)
قَالَ أَصْحَابُ رَسُولِ اللهِ ﷺ لِرَسُولِ اللهِ ﷺ إِنَّ أَهْلَ الْكِتَابِ يُسَلِّمُونَ عَلَيْنَا فَكَيْفَ نَقُولُ؟ قَالَ قُولُوا وَعَلَيْكُمْ