Prayer: Immediately Replying to Salaam
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1124
One must immediately reply to salaam, irrespective of whether he is performing prayers or not. If a person intentionally or due to forgetfulness delays his reply to salaam to the extent that were he to reply to it, it would not be considered a reply to that initial salaam, then in the event that he is performing prayers, he must not reply; and if he is not performing prayers, replying is not obligatory.