I have seen many quotes by Gen. Lee, but, if true, this one shows an almost prophetic wisdom:

"They do not know what they say. If it came to a conflict of arms, the war will last at least four years. Northern politicians will not appreciate the determination and pluck of the South, and Southern politicians do not appreciate the numbers, resources, and patient perseverance of the North. Both sides forget that we are all Americans. I foresee that our country will pass through a terrible ordeal, a necessary expiation, perhaps, for our national sins."-----Robert E. Lee, May 5, 1861.

There are numerous uses of this quote on Civil War Roundtable sites, famous quote sites, etc., but no one sources the quote.
I saw that this quote was used in a lesson plan by U. of Vermont students, but again, no source. I do not doubt the quote, just wondering
(due to the date), where it's coming from......any help appreciated!