Well, I can't help with an original source either, but I can lend a little substance to the probability it was said. Lee's relative by marriage, John Augustine Washington, was killed at Cheat Mountain by a Union picket, and Lee blamed Loring for the death. This relation was the great-grand nephew of George Washington, and the last member of the Washington family to own Mt Vernon - he was a very big deal in Mary's family as they very much revered anything and all things Washington. I believe the subject of Loring's promotion came up with Lee and he was supposed to have said this but to whom and when I don't know. And, as this is an unusually candid thing for Lee to say, almost out of character, it should be noted that his son Rooney was with Washington and barely escaped being killed himself. Lee might have been a little more than just unhappy with Loring!