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.