1. When someone conveys the salaam of a person to you, then you should reply by saying “Alaika Wa Alaihis Salaam”. However, if one replies by merely saying “Wa alaikumus Salaam”, it will suffice.
عن غالب قال إنا لجلوس بباب الحسن إذ جاء رجل فقال حدثنى أبى عن جدى قال بعثنى أبى إلى رسول الله صلى الله عليه وسلم فقال ائته فأقرئه السلام. قال فأتيته فقلت إن أبى يقرئك السلام. فقال: عليك وعلى أبيك السلام. (ابو داود رقم 5233)
Hadhrat Ghaalib (Rahimahullah) reports: “We were once seated at the door of (the home of) Hadhrat Hasan Basri (Rahimahullah) when a person came and said: ‘My father related the following incident from my grandfather. He (my grandfather) said: ‘My father once sent me to Rasulullah (Sallallahu Alaihi Wasallam) saying, ‘Go to Rasulullah (Sallallahu Alaihi Wasallam) and convey my salaams to him.’ When I reached him I conveyed the salaams of my father saying, ‘My father conveys salaams to you.’ Rasulullah (Sallallahu Alaihi Wasallam) replied: “Alaika Wa ala abeeka salaam (i.e. May salaam (peace) descend upon you and upon your father).’”