From the 1974 movie The Conversation, written and directed by Francis Ford Coppola. From IMDb:
Harry Caul (Gene Hackman), a private surveillance expert in San Francisco, has been hired by the Director to follow and record the actions of his wife, Ann (Cindy Williams), and her lover, Marc (Frederic Forrest). Using three separate microphones, Caul and his associates follow the couple around a park. Later, he pieces together a conversation that clearly indicates the two are in a relationship and that they fear being found out by the Director.
In this scene, Caul goes to the hotel that Ann and Marc mentioned as meeting place in their conversation. He rents an adjoining room and inserts a microphone through the wall to listen to the conversation.
This moment ranks right up there among the most visually arresting images in cinema history:
The original April 1974 NYT review of the movie by Vincent Canby.
Do you remember this scene from The Conversation?