James Earl Jones is the well-known voice of Darth Vader, but it was David Prowse who was under the helmet on set. During the filming of The Empire Strikes Back, Prowse allegedly leaked nine separate tidbits to the press about the movie, per Empire. In turn, George Lucas cleverly decided not to give him the “I am your father” line during the historical scene. Instead, he said “Obi-Wan Kenobi is your father” to Mark Hamill.
Article source: https://www.cinemablend.com/news/2546776/why-george-lucas-kept-darth-vaders-empire-strikes-back-twist-from-the-actor