you're telling me
I'm well aware of that, as in She's a terrific dancer.—You're telling me! I taught her how, or You're telling me, the prices are sky-high here. [Early 1900s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.