Narrated `Aisha
A group of Jews came to Allah's Messenger (ﷺ) and said, "As-samu 'Alaika " (Death be on you), and I understood it and said to them, "Alaikum AsSamu wa-l-la'na (Death and curse be on you)." Allah's Apostle said, "Be calm! O `Aisha, for Allah loves that one should be kind and lenient in all matters." I said. "O Allah's Messenger (ﷺ)! Haven't you heard what they have said?" Allah's Messenger (ﷺ) said, "I have (already) said (to them), 'Alaikum (upon you).' "
Narrated `Abdullah bin `Umar
Allah's Messenger (ﷺ) said, "When the Jews greet you, they usually say, 'As-Samu 'alaikum (Death be on you),' so you should say (in reply to them), 'Wa'alaikum (And on you).
Narrated Anas bin Malik
the Prophet (ﷺ) said, "If the people of the Scripture greet you, then you should say (in reply), 'Wa'alaikum (And on you).' "