"You say that!"
Translation:Das sagst du!
It could be either indicative or imperative -- but if it's imperative, it's emphatic, because Pronouns are usually left out in the imperative otherwise.
So "YOU say that!", with the pronoun, is more emphatic that the speaker wants THE LISTENER (and not someone else) to say the thing; in German, that would be Sag du das!, also with the pronoun explicitly added to an imperative sentence.