“McTamany knocked out the longest hit that has yet been made on the new grounds, sending the ball over the left-field fence, 70 feet inside the foul pole.” The Buffalo Expresss, July 11, 1892, p8 (Associated Press)
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1904. “[Deacon] McGuire did better with a double to the left foul pole” (The Washington Post, Sept. 21; Peter Morris).