Musnad Ahmad Hadith 13459

It is narrated from Anas (may Allah be pleased with him) that a Jew, while passing by the Prophet (peace be upon him) and the companions (may Allah be pleased with them), greeted them by saying "As-saam alaikum" (death be upon you). The Prophet (peace be upon him) said to the companions (may Allah be pleased with them), "Do you know what he said?" They replied, "O Messenger of Allah (peace be upon him)! He greeted (gave salaam)." The Prophet (peace be upon him) said, "No, he said this." He then said, "Bring him to me," and asked him, "Did you say 'As-saam alaikum'?" When he admitted it, the Prophet (peace be upon him) said, "When any 'People of the Book' greet you, just reply with 'Wa alaik' (and upon you)."