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: There is a problem here, since this quote is also at times attributed to his father, John Adams, with the following source reference:

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