Volume 9, Book 84, Number 60:
Narrated Anas bin Malik:
A Jew passed by Allah’s Apostle and said, “As-Samu ‘Alaika.” Allah’s Apostle said in reply, “We
‘Alaika.” Allah’s Apostle then said to his companions, “Do you know what he (the Jew) has said? He
said, ‘As-Samu ‘Alaika.'” They said, “O Allah’s Apostle! Shall we kill him?” The Prophet, said, “No.
When the people of the Book greet you, say: ‘Wa ‘Alaikum.'”