No, you must not! Make sure you ask someone before using them as a reference on an application. This is professional politeness that shows the individual you value them. Explain why you are requesting a recommendation and ask if they are comfortable sending you a letter. Make your request during a time when they are available, such as during a professor's office hours. To use someone as a reference, you must first seek their permission to ensure that they agree to vouch for your talents and abilities.