From the Fool in King Lear:
Have more than thou showest, Speak less than thou knowest, Lend less than thou owest, Ride more than thou goest, Learn more than thou trowest, Set less than thou throwest; Leave thy drink and thy whore, And keep in-a-door, And thou shalt have more Than two tens to a score.
Trow (v.) think, expect, believe
I love the line, “speak less than thou knowest.”