Brewer's: Still Waters Run Deep
Silent and quiet conspirators or traitors are most dangerous; barking dogs never bite; the fox barks not when he would steal the lamb.
Smooth runs the water where the brook is deep: And in his simple show he harbours treason. The fox barks not when he would steal the lamb; No, no, my sovereign, Gloucester is a man Unsounded yet, and full of deep deceit.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894