1
general::
Also, you said it. I totally agree with what you said, as in What a relief that Brian didn't get hurt.? You can say that again! or This is a huge house.? You said it. [Colloquial; first half of 1900s]
American Heritage Idioms
2
general::
Phrase(s): You can say that again! [and] You said it!
Inf. That is true.; You are correct. (The word that is emphasized.) • Mary: It sure is hot today. Jane: You can say that again! • Bill: This cake is yummy! Bob: You said it!
McGrawhill's American Idioms And Phrasal Verbs