“No particular mental effort will be required this year for the baseball fans to carry in mind since the season of 1902 ended. But three alterations have been made, and one of these relates to the pitcher’s mound, and the base lines, requiring the base lines at the plate to be perfectly level, and demanding a gradual slope from the mound to the plate and base lines. The mound may not be more than 15 inches higher than the plate and the base lines.” New Orleans Item, April 12, 1903, p12
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1907 (New York Evening Journal, May 4; Edward J. Nichols).