In researching an answer for another question, I came across this claim on a NASA history page:
On September 2  Paine called a press conference to announce NASA's interim operating plan for fiscal 1971. Cutbacks and stretchouts in most major programs were the order of the day, among them the cancellation of two more Apollo missions, 15 and 19. The remaining missions, redesignated Apollo 14- 17, would be flown at six-month intervals in 1971 and 1972. After that would come the three manned Skylab missions (which under earlier schedules had interrupted the Apollo program for two years).
I have attempted to find such an "earlier schedule" which shows the moon landings interrupted by Skylab. NASA's official history of Skylab describes dozens of proposals by the Apollo Applications Program (AAP), which eventually led to the development of Skylab. Most of the proposals described never advanced beyond the AAP, including delaying the lunar missions until after a station was built, a Gemini-Titan-based station, a telescope mounted in a LM shell, and a station built from spent Saturn 3rd stage tanks. However, there is no evidence that any of these proposals were approved or acted upon outside the AAP office.
Is there evidence of approval outside the AAP (e.g. by NASA administration, the White House, or Congress) to interrupt the Apollo lunar program for two years for Skylab?