“Buck Ewing and Tim Keefe agree that Beckley, of the Pittsburghs, is a natural hitter, but Keefe thinks that Beckley will not hit safe so often when the League pitchers thoroughly size him up.” Pittsburgh Post, August 29, 1888, p6. (Original source given as New York Press)
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1893. “Base ball lost its most natural hitter, when big Dave [Orr] retired” (William M. Rankin, The Sporting News, Dec. 23; Peter Morris).