Martha Washington destroyed all her correspondence with George before her death. Only a few letters have ever surfaced because of this. As I was cruising around Mt. Vernon's website earlier, looking for information my smallpox post, I happened to notice this one, that was found and purchased in 2003:
[Mount Vernon, 30 March 1767]
It was with very great pleasure I see in your letter that you got safely down [to Williamsburg.] we are all very well at this time but it still is rainney and wett I am sorry you will not be at home soon as I expected you I had reather my sister woud not come up so soon, as May woud be much plasenter time than april we wrote to you last post as I have nothing new to tell you I must conclude my self
your most Affectionate
You can find find two others through the Papers of George Washington at the University of Virginia. I thought I'd post one from George to Martha for you from this site:
Phila. June 23d 1775.
As I am within a few Minutes of leaving this City, I could not think of departing from it without dropping you a line; especially as I do not know whether it may be in my power to write again till I get to the Camp at Boston—I go fully trusting in that Providence, which has been more bountiful to me than I deserve, & in full confidence of a happy meeting with you sometime in the Fall—I have not time to add more, as I am surrounded with Company to take leave of me—I retain an unalterable affection for you, which neither time or distance can change, my best love to Jack & Nelly, & regard for the rest of the Family concludes me with the utmost truth & sincerety Yr entire