This is true. Brian Mast is the representative for the 21st District in Florida. On Friday, October 13th, 2023, Mast wore his military uniform from his time serving in the Israel Defense Forces (IDF) to Capitol Hill. His did this following Hamas's surprise attack on Israel the weekend prior.
That same day, Mast posted to his Twitter account with photos of him in his uniform being interviewed. In this post he writes, "As the only member to serve with both the United States Army and the Israel Dense Forces, I will always stand with Israel."