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Duas for replying to a non-muslim's greeting.
A short, courteous reply from the Sunnah

The Prophet ﷺ told the believers to spread the greeting of salam among themselves, to those they know and those they do not (Sahih Muslim 54, Sahih al-Bukhari 12). When the People of the Book greet a Muslim with salam, he taught the reply: Wa 'alaykum.

1 sourced Du'a Arabic, transliteration, meaning, and source

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Replying to the greeting of the People of the Book

إذَا سَلَّمَ عَلَيْكُمْ أَهْلُ الْكِتَابِ فَقُولُوا: وَعَلَيْكُمْ

Idha sallama 'alaykum ahlu-l-kitabi faqulu: Wa 'alaykum

“When the People of the Book greet you with salam, reply: Wa 'alaykum (and upon you).”

Sahih al-Bukhari 6258

The reply to say is simply: Wa 'alaykum.

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