it lies on page 251 of Mitt Romney’s book, No Apology:
During my campaign for governor, I decided to spend a day every few weeks doing the jobs of other people in Massachusetts. Among other jobs, I cooked sausages at Fenway Park, worked on asphalt paving crew, stacked bales of hay on a farm, volunteered in an emergency room, served food at a nursing home, and worked as a child-care assistant. I’m often asked which was the hardest job – it’s child care, by a mile.
One day I gathered trash as a garbage collector. I stood on that little platform at the back of the truck, holding on as the driver navigated his way through the narrow streets of Boston. As we pulled up to traffic lights, I noticed that the shoppers and businesspeople who were standing only a few feet from me didn’t even see me. It was as if I was invisible. Perhaps it was because a lot of us don’t think garbage men are worthy of notice; I disagree – anyone who works that hard deserves our respect. – I wasn’t a particularly good garbage collector: at one point, after filling the trough at the back of the truck, I pulled the wrong hydraulic lever. Instead of pushing the load into the truck, I dumped it onto the street. Maybe the suits didn’t notice me, but the guys at the construction site sure did: “Nice job, Mitt,” they called. “Why don’t you find an easier job?” And then they good-naturedly came down and helped me pick up my mess. Here’s Romney working as a garbage collector: 
Picture of Romney operating hydraulic system without license and dumping rubbish on road without a permit.
