My employer pays me regular 40 hour pays checks. anything over 40 he pays cash for. how do i stop this?

Recently I had to go into the place that I work on a Saturday and he said he was only going to pay me regular time for it. isn't that illegal and what can I do to get Overtime because it runs me about 8 hours over what I usually work. Should I TRY to get a copy of my time card and then keep my pay that was paid in cash? I know paying in cash is illegal in the first place, would that end his Business if I go through with trying to get paid that Overtime if I have to go to the Labor Board? If I go to the labor board will they ask me to produce my previous paychecks and if they decide to go that route...am I going to get penalized for it?