Sahih Ibn Khuzaymah

Sayyiduna Samurah bin Jundub (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings of Allah be upon him) commanded us to reply to the greetings (salaam) of our imams, to have mutual love and affection, and to greet one another with salaam. Imam Abu Bakr (may Allah have mercy on him) says that Allah, the Blessed and Exalted, says « وَإِذَا حُيِّيتُم بِتَحِيَّةٍ فَحَيُّوا بِأَحْسَنَ مِنْهَا أَوْ رُدُّوهَا » [ سورة النساء : 86 ] "And when you are greeted with a greeting, greet with one better than it or return it (in like manner)." And in the narration of Sayyiduna Jabir bin Samurah (may Allah be pleased with them both), it is mentioned that then he should greet those on his right side and those on his left side. This is evidence that when the imam says salaam to his right at the end of the prayer, he is greeting the people on his right, and when he says salaam to his left, he is greeting the people on his left. And Allah Almighty has commanded to reply to the salaam of a Muslim person. The statement of Allah Almighty is « وَإِذَا حُيِّيتُم بِتَحِيَّةٍ فَحَيُّوا بِأَحْسَنَ مِنْهَا أَوْ رُدُّوهَا » [ سورة النساء : 86 ] "And when you are greeted with a greeting, greet with one better than it or return it (in like manner)." Therefore, it is obligatory for the follower (muqtadi) to reply to the salaam of the imam, because when the imam says salaam at the end of the prayer, he is greeting the followers.