Here it is:
"Within twenty years we can peacefully colonize the negro and give him our language, literature, religion, and system of government under conditions in which he can rise to the full measure of manhood. This he can never do here. We can never attain the ideal union our fathers dreamed of, with millions of an alien, inferior race among us, whose assimilation is neither possible nor desirable."
-- A. Lincoln, in an address given in Washington D.C. (found in in Basler, Collected Works, Volume V, pages 371-375)
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Lincoln never said it.
It's from a novel, published in 1904 ... "The Clansman," by Tom Dixon
Cheers,
Richard F.