It's called "First Squad, First Platoon," and it's based on a story he wrote about his experiences in World War II. It's been in his daughter's possession for decades and she finally decided to submit it for publication. 

"The story is set on Leyte Island in “heavy jungle foliage” and a “hostile rain that caked mud on weapons, uniforms, equipment.” Each of the five chapters in the 11,000-word story is about a different soldier and how they died."

Unfortunately, it will cost you if you want to read it. The Strand Magazine has the rights and is selling just the story for $12 U.S., or extra fees if you want a year or more subscription to their publication. The Strand is a magazine dedicated to putting out previously unknown or unpublished material by famous writers, including Mark Twain, John Steinbeck, H.G. Wells, F. Scott Fitzgerald and Agatha Christie. 

A WWII story by The Twilight Zone’s Rod Serling is published for the first time