If your employer made this claim to someone other than you (customer, friend, coworker, supervisor, etc.), then you may have a lawsuit for defamation of character against the employer. Otherwise, there is no legal recourse for you.
Employers can be amazing asses, sometimes. Many times, an employee has a claim that they can make -- many times not. Sometimes, you just have to ignore the employer, because, frankly, the employer is a person, too, and he/she may have a tough day, too, and is taking it out on you for no good reason.
You can write a letter to your employer stating that you deny the allegation and that if the employer makes this claim to any third party, that you will take appropriate legal action to defend your interests. However, by doing this, you may just annoy the employer to the point where he/she fires you. So, what you choose to do must be tempered by how badly you need the job.
Hope this helps.
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