Witness (1985)

When Samuel (Lukas Haas), a young Amish boy traveling with his mother Rachel (Kelly McGillis), witnesses the murder of a police officer in a public restroom, he and his mother become the temporary wards of John Book (Harrison Ford), a detective who's been assigned to solve the crime. After suspect lineups and mug-shot books yield nothing, Samuel, in the most memorable scene of the film, recognizes the murderer as a narcotics agent whose picture he sees in the precinct.