رِيَاضُ الصَّالِحِينَ
Riyad as-Salihin 867
The Book of Greetings
Greeting Non-Muslims and the Prohibition of Taking Initiative in Doing So
وَعَنْ أَنَسٍ رَضِيَ اللَّهُ عَنْهُ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: "إِذَا سَلَّمَ عَلَيْكُمْ أَهْلُ الْكِتَابِ فَقُولُوا: وَعَلَيْكُمْ” ((مُتَّفَقٌ عَلَيْهِ))
Anas (May Allah be pleased with him) reported: Messenger of Allah (ﷺ) said, "When the people of the Book greet you, respond with: 'Wa 'alaikum' (And upon you)". [Al-Bukhari and Muslim].
Riyad as-Salihin, 867