It was in fact, "No. I am your father." However, the trilogy's most famous line was never actually delivered by Vader - on the set, he really said: "Obi-Wan killed your father," but the line was secretly re-dubbed later. Luke responds in horror: "No! No! That's not true. That's impossible."
The misquoted line, as "Luke, Luke, I am your father" was heard in "Tommy Boy," by the title character Tommy (Chris Farley) as he goofed off in front of an electric fan. It was also spoofed in "Toy Story 2" by Buzz Lightyear (voice of Tim Allen) who was confronted in an elevator shaft and told by arch-enemy nemesis Emperor Zurg (voice of Andrew Stanton): "I am your father," followed by Buzz's anguished scream: "Nooooo!"
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