(4) باب إِذَا عَرَّضَ الذِّمِّيُّ وَغَيْرُهُ بِسَبِّ النَّبِيِّ صلى الله عليه وسلم وَلَمْ يُصَرِّحْ نَحْوَ قَوْلِهِ السَّامُ عَلَيْكَ
Narrated Anas bin Malik:
A Jew passed by Allah's Messenger (ﷺ) and said, "As-Samu 'Alaika." Allah's Messenger (ﷺ) said in reply, "We 'Alaika." Allah's Messenger (ﷺ) then said to his companions, "Do you know what he (the Jew) has said? He said, 'As-Samu 'Alaika.'" They said, "O Allah's Messenger (ﷺ)! Shall we kill him?" The Prophet, said, "No. When the people of the Book greet you, say: 'Wa 'Alaikum.'"
Reference: Sahih al-Bukhari 6926
In-book reference: Book 88, Hadith 8
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 9, Book 84, Hadith 60