Pitchers and Catchers Report (1887)

Baseball in the Northwest will make a beginning for the season about the first of the next month, when the pitchers and catchers will report for duty and begin active practice, probably in the Washington rink.” Minneapolis Tribune, March 7, 1887, p4

First use specifically mentioning spring training:

The local club {Toldeo Mudhens} will do its spring training at Columbus, Ga. Armour has ordered the pitchers and catchers to report here March 6.Hutchinson Daily Gazette, January 18, 1909, p3

Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1927. “The Phils did announce their spring training plans—pitchers and catchers will report at the Bradenton camp in Florida, a week earlier than the infielders and outfielders, which will be in late February” (The Sporting News, Jan. 13; Peter Morris).