nói có sách, mách có chứng

Vietnamese edit

(literal) if you're gonna say something, cite your books, and if you're gonna tattle, show your proof
you ought to always back up your claims with evidence
- (Can we date this quote?), Thôn Cô, Vợ xấu chồng mù [Ugly Wife, Blind Husband]:
  “Thưa mẹ, nói có sách mách có chứng, phiền mẹ đưa chứng cứ rồi muốn nói con thế nào cũng được.”
  “Mother, extraordinary claims require extraordinary evidence, so would you be kind enough to provide your evidence, then you can argue with me to your heart’s content.”