Reply to delegation from the National Union League approving and endorsing "the nominations made by the Union National Convention at Baltimore." New York Times, Herald, and Tribune (10 June 1864) Collected Works of Abraham Lincoln. Volume 7 http://quod.lib.umich.edu/cgi/t/text/text-idx?c=lincoln;rgn=div1;view=text;idno=lincoln7;node=lincoln7%3A852
To a delegation of the National Union League who congratulated him on his nomination as the Republican candidate for President, June 9, 1864. As given by J. F. Rhodes—Hist. of the U. S. from the Compromise of 1850, Volume IV, p. 370. Same in Nicolay and Hay Lincoln's Complete Works, Volume II, p. 532. Different version in Appleton's Cyclopedia. Raymond—Life and Public Services of Abraham Lincoln, Chapter XVIII, p. 500. (Ed. 1865) says Lincoln quotes an old Dutch farmer, "It was best not to swap horses when crossing a stream".
Variant: I do not allow myself to suppose that either the convention or the League, have concluded to decide that I am either the greatest or the best man in America, but rather they have concluded it is not best to swap horses while crossing the river, and have further concluded that I am not so poor a horse that they might not make a botch of it in trying to swap. note
Source: http://quod.lib.umich.edu/cgi/t/text/text-idx?c=lincoln;rgn=div1;view=text;idno=lincoln7;node=lincoln7%3A852 Collected Works of Abraham Lincoln. Volume 7