Where does the idea come from that the KJV is the proper bible to be used?

I have heard (mostly from the older generations) that the KJV is the only bible to be used, and all the other ones are changing the bible. Where does this idea come from? Is it a churches teaching or the individual opinions of people? I have learned from my studies that it is probably the least reliable bible, so I was wondering why people set their minds so much upon the KJV.