Agreement needed prior to approaching potential partner for side project?

I have an idea for a small SaaS product. I've done some initial market research and the field looks promising (in Midwest terms, maybe not in Silicon Valley terms). I've created some mock-ups and wire frames and I think it could be a great side project that -may- have the opportunity to make money some day.

I have a friend who I would like to invite to partner up and share the coding work. Since the project is merely thoughts and sketches right now, there isn't anything to really protect (ideas are a dime a dozen, execution is what matters) but since this project has the -potential- to be profitable some day, is there any type of agreement or paperwork I should have both of us agree to before bringing it up to him?

I have read so many stories of founders getting burned by not having the right paperwork in place and things getting ugly when the stuff hits the fan. Does the fact that we wouldn't be actively seeking to make money initially mean there is no worries?

Just want to get my ducks in a row before I end up being the next cautionary tale.