Asking Permission

Bukhari :: Book 8 :: Volume 74 :: Hadith 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)."