Upvote:-1
If anything, your ID card. Passport, of course, does the job as well. In most cases, nothing is required, but just in case its always good to have one of the two on you.
Upvote:7
Short answer: Your passport or EU-based national identity card. However, chances are slim that you will be checked, but it is not uncommon and you are obliged to show an accepted travel document.
Usually those checkups are done in the context of some specific objective (drugs, wanted criminals, or another multinational collaborative effort by the authorities) and often if you are not suspected anything will do (e.g. driver's license, residence permit, bank card, etc.). If however they choose to go formal on you, you need to have a proper travel document stating your resident status and citizenship.
There is a website stating what official identity cards are issued per country.