“‘The pitcher has to cut the heart of the plate,’ said the player, ‘and the umpires will not call a strike on a curve ball.’” The Daily Courier [Syracuse], April 27, 1887, p4
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1897. “I’m going to shove it plump over the heart of the plate” (pitcher John Clarkson, quoted in The Sporting Life, Oct. 30).