Right for references

Maybe anyone can help me with my question: Is there a right for references? More concrete: When you have worked for a company, equally as an employe or rather as en external company, do you have the right to show this work as your reference? Even when you have done only a small part of a project, do you have the right to say, this or that company is my customer? I´m not quite sure. Please help, thanks a lot in advance…