I have always thought that all men should be free; but if any should be slaves it should be first those who desire it for themselves, and secondly those who desire it for others. Whenever [I] hear any one arguing for slavery I feel a strong impulse to see it tried on him personally.

Abraham Lincoln (1809-1865) American lawyer, politician, US President (1861-65)
Speech (1865-03-17) to the 104th Indiana Regiment, Indianapolis
    (Source)

Lincoln was speaking on reports that Confederate Army was drafting Black slaves to fight in their ranks. The above is the text from Lincolns autograph draft, and is most well known. The draft includes the words (scratched out) "any one arguing for slavery, even a preacher, I feel ...."

See also Lincoln (1854).

The following version was from newspaper reports the next day in the New York Herald and New York Tribune:

While I have often said that all men ought to be free, yet I would allow those colored persons to be slaves who want to be; and next to them those white persons who argue in favor of making other people slaves. (Applause.) I am in favor of giving an opportunity to such white men to try it on for themselves.

The "arguing for slavery" quote was mentioned in a speech by Jimmy Carter before the Indian Parliament (1978-01-02), in the context of those wealthy people who say that democracy is of no value to the poor.