The quote is dated in the summer of 1870, and it was at a gathering of old generals at White Sulphur Springs, Virginia. General Rosecrans was there and he had rubbed General Lee the wrong way.
The whole quote can be found in....
"The life and letters of Robert Lewis Dabney" By Thomas Cary Johnson page 499
http://books.google.com/books?id=oKbyqOdg9RQC&printsec=frontcover&dq=The+life+and+letters+of+Robert+Lewis+Dabney&cd=1#v=onepage&q=&f=false