You're telling me!
American SlangDefinition
An exclamation of emphatic agreement, often implying the point is obvious or already known to the speaker.
Examples
"After he groaned, 'This humidity is unbearable,' she shot back, 'You're telling me!'"
"When his coworker said, 'These meetings drag on forever,' he replied, 'You're telling me!'"
"Hearing 'Rent keeps going up every year,' she rolled her eyes and said, 'You're telling me!'"