The Ruling regarding Salām in General
Issue No.1019- It is obligatory to answer another’s Salām even though one is offering prayer. Saying Salām is recommended and one should reply to it in a way that it may be counted as the reply to Salām. That is to say, if he delays the reply so much that it may not regarded the reply to that Salām, then he has committed a haram act; it would no longer be obligatory to reply to it though.