Sahih Bukhari
Asking Permission
Volume: 008 / Book Number: 074 / Hadith Number: 274
Narrated 'Abdullah bin 'Umar:
Allah's Apostle 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)."