Lincoln made many speeches, letters, and claims to declare how against he is towards slavery. He explains in each source how he doesn't support slavery.
October 16, 1854: Speech at Peoria, Illinois "I can not but hate [the declared indifference for slavery's spread]. I hate it because of the monstrous injustice of slavery itself. I hate it because it deprives our republican example of its just influence in the world -- enables the enemies of free institutions, with plausibility, to taunt us as hypocrites -- causes the real friends of freedom to doubt our sincerity, and especially because it forces so many really good men amongst ourselves into an open war with the very fundamental principles of civil liberty -- criticising [sic] the Declaration of Independence, and insisting that there is no right principle of action but self-interest."
August 24, 1855: Letter to Lincoln's friend Joshua Speed "You know I dislike slavery; and you fully admit the abstract wrong of it. So far there is no cause of difference. But you say that sooner than yield your legal right to the slave -- especially at the bidding of those who are not themselves interested, you would see the Union dissolved. I am not aware that any one is bidding you to yield that right; very certainly I am not. I leave that matter entirely to yourself. I also acknowledge your rights and my obligations, under the constitution, in regard to your slaves. I confess I hate to see the poor creatures hunted down, and caught, and carried back to their stripes, and unrewarded toils; but I bite my lip and keep quiet. In 1841 you and I had together a tedious low-water trip, on a Steam Boat from Louisville to St. Louis. You may remember, as I well do, that from Louisville to the mouth of the Ohio there were, on board, ten or a dozen slaves, shackled together with irons. That sight was a continual torment to me; and I see something like it every time I touch the Ohio, or any other slave-border. It is hardly fair to you to assume, that I have no interest in a thing which has, and continually exercises, the power of making me miserable. You ought rather to appreciate how much the great body of the Northern people do crucify their feelings, in order to maintain their loyalty to the constitution and the Union."