I believe he actually said, "Obi-Wan killed your father."
I'll go find some proof.
*In the original film, Skywalker is told by Obi-Wan Kenobi that his father, Anakin Skywalker, was betrayed and murdered by Kenobi's own apprentice, Darth Vader. However, in The Empire Strikes Back, Vader himself reveals that he is actually Anakin. According to director Irvin Kershner, the shooting script had a false page with Vader's dialogue implicating Kenobi as the murderer of Luke's father, but at the time, only he, George Lucas, screenwriter Lawrence Kasdan, and producer Gary Kurtz were privy to the knowledge that Vader and Anakin were to be the same person.
Moments before the scene was filmed, Mark Hamill, the actor playing Luke, was taken aside privately and told the truth by Kershner, who encouraged him to ignore the dialogue Prowse was speaking and "use your own rhythm compared to what he's doing". During filming, Prowse performed the false lines, while the real lines were later spoken by James Earl Jones in post production – the majority of the cast, including Prowse, were not aware of this plot twist until the first screening of the finished film; at the premiere of the film, Prowse was taken aback, and spoke privately to Kershner later, saying that he (in Kershner's words) "would have done it differently" had he known of the twist. In later years, Prowse claimed he would sometimes deliver joke versions of his lines in Empire and Jedi, knowing that the vocal takes would not be used.*