One if by land, and two if by sea

The words used by Henry Wadsworth Longfellow in his poem “Paul Revere's Ride” to describe the signal used to guide the “midnight ride of Paul Revere” at the start of the Revolutionary War. Revere had ordered two lanterns to be placed in a Boston church tower to warn his confederates that the British were on the move. Longfellow embellished the story a little.