Someone sent a text message to the unit leader to slander me. What should I do?

It's slander, but it's not true. You can talk to the unit leader and ask him to investigate. If there is no such thing after the investigation, it will prove that this person is indeed a libel crime. If the leader doesn't name people, you can ask the leader to punish the slanderers who dare not name them in front of everyone when there are many people in the unit. That person must know that he (she) won't dare to do this in the future.