Posterity - you will never know how much it has cost my generation to preserve your freedom. I hope you will make good use of it.

Source: John Adams, letter to Abigail Adams from Philadelphia, April 26, 1777; “Letters of John Adams, Addressed to His Wife,” Edited by His Grandson, Charles Francis Adams, Volume I, 1841

There are internet sources that attribute this saying to John Quincy Adams (6th President of the USA). The two Adams’s are sometimes confused.