George Washington in a letter to Robert Morris, April 12, 1786.
I hope it will not be conceived from these observations, that it is my wish to hold the unhappy people who are the subject of this letter, in slavery. I can only say that there is not a man living who wishes more sincerely than I do, to see a plan adopted for the abolition of it—but there is only one proper and effectual mode by which it can be accomplished, & that is by Legislative authority: and this, as far as my suffrage will go, shall never be wanting.
Source: The Founders Archives