Nope, not Stanton, Michael. Congratulations to Rob! Indeed it was Horace Greeley!! The quote came from a speech given by Greeley. I do not have a specific date for the speech. Good thinking, Rob!
My source for the quote: Lincoln as I Knew Him: Gossip, Tributes, and Revelations from His Best Friends and Worst Enemies edited by Harold Holzer, p. 111.