Hadith 17:384
Ibn ‘Ulayyah narrated to us, from Abu Hayyan, from ‘Abd al-Rahman al-Rahhal, who said: When al-Rabi‘ ibn Khuthaym would return a greeting, he would say, "Wa ‘alaykum" (And upon you), meaning he intended to return [the greeting] to the one who greeted him.
حدثنا بن علية عن أبي حيان عن عبد الرحمن الرحال قال كان الربيع بن خثيم إذا رد يقول وعليكم يعني ينوي الرد على ما سلم عليه