The cruellest lies are often told in silence. A man may have sat in a room for hours and not opened his teeth, and yet come out of that room a disloyal friend or a vile calumniator.

Robert Louis Stevenson (1850-1894) Scottish essayist, novelist, poet

Essay (1879-05), “The Truth of Intercourse,” Cornhill Magazine, Vol. 39
    (Source)

Collected "Virginibus Puerisque, Part 4" in Virginibus Puerisque and Other Papers, ch. 1, part 4 (1881).