This is part of the closing narration of episode 16, season 3—titled “Nothing in the Dark”—from /The Twilight Zone/. It was spoken by Rod Serling but written by George Clayton Johnson. See the Wikipedia articles for Johnson (http://bit.ly/2rBfOMg) and “Nothing in the Dark” (http://bit.ly/2sRhDJf), especially the section for “Closing narration.” There are a number of confusing YouTube videos of this episode (featuring a very young Robert Redford), but I found the closing narration in the middle of one of them (http://bit.ly/2sUf1d9), at 22:58.