It is considered as a famous issue that almost majority of people in my country (Iran) say that it is Wajib (compulsory) to reply the person who says Salam to you. I.e. if somebody tell you "a-Salamon-Alaikom", then it would be considered as a Wijib practice for you to tell him or her "Alaikum-aSSalam".
I was wondering if it is an Islamic rule. If so, is there any related Qur'anic verse or hadith (tradition) concerning it?