Situation: I visit a customer. That customer paid for a given numbers of hours for on-site support. To make everything very clear I'd like to have something to certify what was done, when, how long...

This way I have a proof if at one point that customer say that I did'nt respect my engagement in all my visits.

So what kind of documents do you guys have to control this?

Or do you have web sites that could show samples for those?