How do scholars deduce that Jumuah is mandatory only for urban/settled people?

In many fiqh books we read that among the conditions for Jumuah some scholars consider being a citizen of city or being settled as a condition for Jumuah to be a fard (mandatory) prayer on a male person who has no valid excuse. However there seem to be a valid dispute whether this condition is correct or not! As far as I know the Prophet () has never prayed Jumuah if he was traveling even if he had enough people in his company to lead a congregation. I'd like to know a bit more about these conditions and the evidence of both views: only settled people must pray Jumuah and those saying even people living in a rural area are asked to pray it. And what exactly defines a rural area where one might not pray Jumuah (according to those who consider it as not mandatory for males living there)?