u/Key_Paramedic_951

I saw somewhere that it's haram (?) when a non-muslim says assalamu'alaikum to you and you reply back with wa'alaikum assalam. Instead, you should reply with 'wa'alaykum' only. Is this a sunni thing or do we have this belief too? And if it's part of our belief system too, can someone explain why it's haram?