"You said it” meaning “I concur”.
How old is this? For some reason I can image Dean Martin saying it in the 1950s.
I can imagine seeing it in an early 20th century comic strip. Is that a significant antedate?
Should I have used a winky? ;)
The OED has it from 1919, and that first entry is from Jargon Book by C.H. Darling, evidently a compilation of slang, etc., so it was widespread enough at that time to have come to the notice of the compiler.