In his letter to the Republican Party, accepting their presidential nomination in 1880. Nowadays a candidate would give a speech in person to the party's convention (and on live television), but at that time it was considered cheesy for a candidate to actually be at the convention -- it would looked like the candidate was too ambitious. But the letter was basically his opening campaign "speech" and would have been reprinted in newspapers and campaign brochures. There's a copy of the full letter here:
