Sahih Bukhari

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