“And yet an effort is made to strengthen the team and not the pitching staff! This is bosh.” Davenport Daily Times, July 13, 1888, p3. (Also appeared in other papers.)
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1894. “The Phillies’ pitching staff is in very bad shape” (The North American [Philadelphia], June 9).