A verbal disagreement, typically delivered as a call-and-response seen in many church sermons, whose meaning and intensity varies upon the speaker--at its most polite, it can be construed as "I respectfully disagree" or "That's what you think"; at its most intense, it means, "Bullshit!" or "You fucking liar!"
Tourist: But everyone gets killed in this town!
Local: Yeah, so sayeth you, dumbass; have you seen how nice this place is?
Mother: Your hair's getting too long. You need a haircut!
(Bald) Son: So sayeth you.