“A brilliant suggestion: Harry Wright should have his men slide feet first instead of head first, as then the basemen would give them the line. When a base runner slides head first an evil, disposed baseman can easily hurt him.” St. Louis Post -Dispatch, May 11, 1886, p5
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1890. “The Toledo men are dirty ball players. They slide feet first. It is contemptible work, and should be called down” (Columbus [Ohio] Post, Apr. 19; Peter Morris).