Herr Starr says it in one of the early issue of Gath Ennis' Preacher.
It's one of the more overrated comic books out there, but still worth a read if you're not easily offended. The characters aren't really well fleshed out.
Also, this exchange, when the Sheriff's Department is investigating a burned-out church:
Sheriff Root: "Ask Me, I reckon it was niggers"
Deputy: "How you reckon that, Sheriff Root?"
Sheriff Root: "Kinda thing they do"
Deputy: "What, burn two hundred people to death, right down to the bone? They do that?"
Sheriff Root: "MARTIAN niggers”