"‘Oh, well, I dunno. But she didn’t look to me like a murderess.”
‘And how do you know what a murderess looks like? Have you ever met one?’
‘Well, I’ve seen them at Madame Tussaud’s.’"
Strong Poison, by Dorothy L. Sayers (copyright 1930; quote is from page 35 of the Hodder & Stoughton trade paperback edition, 2016).